A protest movement that was inspired by the new calculus of the democratic revolutions in Tunisia and Egypt now appears to be following a decidedly old equation: the Sunni-Shiite divide that has riven the Middle East with violence for centuries.

And with Saudi Arabian tanks now in Bahrain to help keep order and many of the nation’s majority Shiites dismayed that their demands for greater influence in the country will not be met anytime soon, the future of this prosperous island looks suddenly contentious and bleak.

“I could see a Sunni extremist blowing himself up during an Ashura celebration” — a major Shiite holiday — “or a Shia going to a Sunni mosque,” said Jasim Husain, a member of the main Shiite opposition political society, al-Wefaq, and a former member of parliament.

“Bahrain is not going to be stable,” he said. “The country has changed forever.”

The protesters occupied Pearl Square in central Manama just days after demonstrations in another square, Cairo’s Tahrir, helped bring down Egyptian President Hosni Mubarak. Those movements were largely youth-driven and mostly secular. So too were the revolution in Tunisia and protests in Yemen, Oman and Kuwait.

The Bahraini protesters’ demands — that the al-Khalifa monarchy give up many of its powers and that the parliament be reorganized to give more power to the 70 percent of the population that is Shiite — proved too much for Saudi Arabia. Its leaders feared the influence of Shiite Iran in the tiny island nation off its coast and worried about Bahrain’s influence on Saudi Arabia’s own Shiite population in the oil-rich Eastern Province.

Shiites in Iraq, Iran, Kuwait and Saudi Arabia took to the streets in recent days to protest the crackdown in Bahrain, further reinforcing the impression among some Bahraini Sunnis that the demands were sectarian, not political, in nature.

Iranian President Mahmoud Ahmadinejad said last week that the Saudi involvement in Bahrain was “foul,” heightening the regional crisis.

Husain said he worried that the charges of sectarianism were self-fulfilling.

“Now I think we are going to see more Iranian influence,” he said. “We had a golden opportunity to overcome our problems in the best way possible, and we have missed it.”

Meanwhile, there are Sunni boycotts of many Shiite businesses, and vice versa. Many here say even friendships between Sunnis and Shiites have changed, at least for now.

Many protesters, meanwhile, still say that all they want is equal opportunity, not sectarian strife.

“I don’t want Iran to come. I don’t want Saudi military to continue in Bahrain. Both is wrong,” said Ahmed Hussein, 35, who works at an aluminum factory and lives in Isa Town, one of the few places in Bahrain where Sunnis and Shiites live together.

He said Saudi Arabia sent its troops to Bahrain because it thought the country’s protests might be contagious.

“If it’s successful, then it’ll transfer to Saudi Arabia,” he said. “The whole world is changing right now.”

The split between Sunnis and Shiites goes back almost 1,400 years, to the early days of Islam. It has spawned many conflicts, including the long and bloody war between Iran and Iraq, and more recently the battles between Sunnis and Shiites in Iraq after the U.S. invasion.

Saudi Arabia’s King Abdullah bin Abdul Aziz announced changes Friday aimed at settling its own nascent protests, most of which have come from the Shiite stronghold of Eastern Province.

Cash and a stronger state, not political reforms, were the top priorities. He expanded security forces by 60,000, raised the monthly minimum wage and gave government workers a pay hike. He also said he would fight corruption.

Residents of the Shiite town of Qatif in Eastern Province, the site of some of the biggest protests, said last week that the king had less to fear from a sectarian revolt inspired by Bahrain than from a democratic movement inspired by Egypt and Tunisia.

Their demographics are different from those in Bahrain, though, where Shiites are a majority of the citizens.

In Bahrain, “what you are seeing will never go away,” Sheikh Isa Qassim, the top Shiite cleric in the country, said Friday in a sermon attended by thousands. “The roots of the problem are still there.”

Meanwhile, the State Department sharpened its rhetoric against Bahrain’s government late Friday, saying it was “deeply troubled” by the arrests of opposition leaders and calling for “security forces to cease violence” in a statement from spokesman Mark Toner.

“A solution to Bahrain’s problems will not be found through security measures,” he said.

Another person was confirmed dead Saturday from the week’s clashes, bringing the death toll to at least eight, according to government and hospital officials. Dozens of people are missing, Bahraini human rights groups said, and it is unclear whether they are in hiding or have been arrested.