Three hundred people marched peacefully through streets filled with charred debris one recent night, waving Bahraini flags and shouting slogans against King Hamad and the United States. Without warning, helmeted riot police in SUVs came screeching up and began firing tear gas at the demonstrators, who panicked and ran.

Screaming women stumbled over their long black abaya robes, old men and children in sandals sprinted as hissing tear gas canisters whizzed past. A group of about 20 teenage boys began throwing molotov cocktails at the police officers.

Eyes watering and coughing up gas, human rights activist Said Yousif al-Muhafdah ran to his car and tapped out two messages reporting news of the clash, in Arabic and English, to his more than 70,000 followers on Twitter.

Violence is rapidly rising in this Persian Gulf island nation, a key U.S. ally that hosts the Navy’s 5th Fleet. The escalation comes about 21 months after dozens of people were killed and thousands arrested when Bahraini security forces brutally quashed an Arab Spring pro-democracy uprising.

Almost every night, security forces clash with marchers demanding that the monarchy be replaced with an elected government. In recent weeks, police have fatally shot two protesters, and a police officer was killed with a homemade bomb.

On Tuesday, Bahrain’s Interior Ministry issued a ban on all public rallies and gatherings, the government’s most aggressive move against protesters since it imposed martial law for three months last year. But rather than ease tensions, the move seems only to have stoked them.

Several protests against the ban were held Friday. Muhafdah tweeted photos of tear gas being used against demonstrators in the village of Bilad al-Qadeem, just west of the capital.

Later in the day, the activist was arrested while trying to photograph an injured protester, and police said he will be charged with violating the ban on public gatherings, Maryam al-Khawaja, who works with Muhafdah at the Bahrain Center for Human Rights but lives in exile in Denmark, said in a telephone interview.

As the standoff intensifies, U.S. policy is coming under fire, criticized by both protesters and the government as feeble and disappointing. U.S. officials say the ruling al-Khalifa family should not be compared to the brutal regimes in Libya and Syria that have killed thousands of their own people. But Washington is under pressure to defend pro-democracy activists who are being systematically arrested and tear-gassed — and even jailed for tweets critical of the government.

The State Department expressed deep concern this week about the protest ban but overall has been more measured in its criticism of Bahrain’s government than it has been with other repressive governments in the region.

“We only want democracy,” Muhafdah said recently. “In the United States, you have a new elected president every four years. But here we are living with a king and the same prime minister for 42 years.”

On the street, protesters said President Obama is not living up to his pledge, made in the last presidential debate, that “America has to stand with democracy.” They said pro-democracy demonstrators are not getting the U.S. support they need because the monarchy allows American warships here, just a few miles from Saudi Arabian oil fields, to help keep vital shipping lanes open and serve as a counter to Iranian belligerence.

“We are a victim,” Muhafdah said, “because we have oil and we have the American 5th Fleet.”

Bahraini officials, meanwhile, complain that the United States has not adequately condemned the protesters, whom they describe as violent and seditious “thugs” influenced by Iran.

Michael H. Posner, U.S. assistant secretary of state for democracy, human rights and labor, said Washington has been actively involved in discussions with both the Bahraini government and the protesters.

“We have been very clear from the get-go that we have a strong national security interest in Bahrain, a 60-year military history, and they are a key ally,” he said. “We are going to continue to engage on that level. But on a parallel track, we are absolutely committed to pursuing a human rights and democracy agenda.”

Shiites vs. Sunnis

Follow Muhafdah for a few days and you understand the forces threatening the government of King Hamad bin Isa al-Khalifa.

On a recent Thursday evening, Muhafdah drove to a rally of more than 2,500 anti-government demonstrators near the capital. When the 30-year-old activist arrived, looking stylish in his gelled hair, jeans and soft-leather loafers, men and women hugged him, kissed him on both cheeks and pushed him to the front. Muhafdah is among the country’s best-known human rights activists, and people treat him like a rock star.

One after another, speakers railed against the monarchy, demanding an elected government, more jobs, less discrimination and a less brutal police force.

The grievance here is partly rooted in the long-standing struggle between the two main branches of Islam, the Shiites and the Sunnis. Shiites constitute the majority of Bahrain’s 1.3 million people, but Muhafdah said Shiites like himself have long been excluded from most top government jobs, including in the army and the police.

Hussain Neamah, 47, dressed in an all-white traditional robe, told the crowd about how police killed his 16-year-old son, Ali, with two blasts of birdshot at close range on Sept. 28.

“He was killed with the regime’s gun,” Neamah said, and the crowd broke into chants of “Yasqot Hamad,” or “Down with Hamad.

When he left the stage, Neamah embraced Muhafdah with moist eyes. Police said that Ali was throwing molotov cocktails and that the officer who shot him was defending himself; Neamah said his son was unarmed.

Muhafdah embraced Neamah and offered a few quiet words of comfort.

“I try to give them support,” the activist said. “They are angry and sad. But this is the cost of our struggle.”

Driving away, Muhafdah said police seem to have been trying to reform since November, when a panel of international investigators who reviewed the government’s crackdown on Arab Spring protesters last year concluded that there was a “culture of impunity” in Bahrain’s security services. The monarchy was internationally condemned for torturing even doctors and nurses detained for treating wounded protesters; several medics remain in prison.

But no senior officials have been fired for the abuse. Muhafdah said the police culture is still brutal. “They are told: ‘The protesters are enemies. They are calling for the regime to fall down. Silence them,’ ” he said.

Police chief Tariq al-Hassan said reforms enacted since last year included a police code of conduct, video cameras in interrogation rooms and other measures. “Are we moving in the right direction? I think we are,” Hassan said. “Are we moving fast enough? I don’t think so.”

Justice Minister Khalid bin Ali al-Khalifa, a member of the royal family, said the government is willing to negotiate with the protesters. Both sides accuse the other of blocking talks.

Khalifa called the nightly street violence “a threat to our national security,” instigated by Iranian-backed revolutionaries who he said want to turn Bahrain into a Shiite theocracy, an accusation that Muhafdah and others deny. The justice minister said that the government would use “all necessary means” to stop the violence and that the royal family would never step aside. “We will stay here. We will live here, and we will die here.”

Birth of a political agitator

Muhafdah didn’t start out as a political agitator. His father and brothers are bankers, and he had been happily working as an insurance salesman. But in 2007, when he was 25, he and a friend went to a human rights workshop at which he heard former prisoners, many of them women, telling stories of being tortured in Bahraini jails.

“I said, ‘This is happening in my country?’ I did not sleep well that night.”

Muhafdah met with Nabeel Rajab, head of the Bahrain Center for Human Rights and the leading rights advocate on the island. Rajab hired Muhafdah to set up Facebook and Twitter accounts and to start posting video to YouTube and photos to Flickr.

Rajab has been arrested many times for leading protests, and in August he was sentenced to three years in prison, a punishment that has energized the demonstrations. Human rights groups and the State Department have called for Rajab to be released.

Muhafdah’s anti-government activities got him fired from his job. He and his wife support their two girls on her salary as a teacher.

He works out of two “offices”: the driver’s seat of his Chevy and a soft leather chair in a coffee shop in one of Bahrain’s many shopping malls. In a typical day, he conducts nonstop media interviews via phone and Skype, meets with other activists and visits wounded protesters.

Through a network of volunteer assistants in 25 villages, Muhafdah is a clearinghouse for information on human rights violations. He said he doesn’t organize protests. That is mainly done by political parties, especially al-Wefaq, the major Shiite group in Bahrain.

Muhafdah attends as many protests as possible and documents police behavior.

On a recent Friday afternoon, the heart of the Muslim weekend, another big protest was called in the capital’s Old Souk shopping district.

By noon, the shops and coffeehouses in the neighborhood were filled with people awaiting word sent by organizers through Twitter about when to step into the streets.

Muhafdah said the protest was organized by a group called the Feb. 14 Youth Coalition, which wants a complete overthrow of the monarchy — a more radical position than Muhafdah supports. (He backs an elected government with a ceremonial monarchy.)

“We have a king who has been killing and torturing his own people,” said Zainab al-Khawaja, 28, a key proponent of the Feb. 14 group who had just been released from prison after serving two months for ripping up a photo of Hamad. “We should have the right to protest against that.”

She and Muhafdah said U.S. policy toward Bahrain was making things worse. They said that since Obama gave a speech in May 2011 criticizing Bahrain’s “brute force,” the United States has been too quiet about the island nation.

“The American government cares more about their political interests than they do about freedom and democracy,” said Khawaja, who earned a bachelor’s degree at Beloit College in Wisconsin. “By supporting the dictatorship here, they are making the people feel hopeless.”

Muhafdah noted that in May the United States agreed to sell Bahrain tens of millions of dollars worth of missile systems and other weapons whose sales the Obama administration had frozen last year over concerns about the island’s crackdown on Arab Spring protesters.

Posner said he has visited Bahrain five times since last year’s violence, both to bolster the security alliance and to call for an end to violence and human rights abuses by police and protesters. But he said that “both sides are dug in” and that he senses an “increased polarization” of society with no immediate hope for negotiation.

“The impasse is going to be resolved by Bahrainis themselves,” he said. “We encourage them to come to the table, but we can’t make that happen.”

‘The change will come’

A few minutes after 3 p.m. that Friday, phones were lighting up with tweets. Muhafdah walked outside, where riot police with shields, batons and tear gas guns stood on almost every corner.

Small groups of people were trying to come together to march, but clusters of police officers stepped in whenever the groups got too big. The hundreds of protesters outnumbered the dozens of police personnel and soon began shouting anti-government slogans.

Police tossed stun grenades that exploded with a deafening bang. The crowd raced down narrow alleys, with police officers rushing after them firing tear gas. Officers in helmets and body armor pushed men and women and doused them with pepper spray.

Muhafdah watched from the sidelines, covering his mouth. He took photos of people being arrested and tweeted them, while plainclothes police took photos and video of him.

The next morning, Muhafdah was back in his coffee shop “office” when Hamad delivered a speech criticizing “foreign interference in our domestic affairs,” a clear reference to Iran, and noted the recent “dangerous escalation” in violence. He called for dialogue but also warned that the government would “criminalize anything that attempts to erode the unity of our nation.”

“They always say we are Iranian spies or American agents — they’ll say anything,” Muhafdah said. “Maybe they will arrest me and think that nobody will talk about human rights anymore. But then another activist would come after me. The change will come. They cannot silence it.”