Protesters throw stones at police during a demonstration in Kabul on June 2 over the country’s deteriorating security situation. (Massoud Hossaini/AP)

Suspected Taliban gunmen ambushed Afghan guards who were in a vehicle headed to work at the U.S.-run Bagram air base, killing at least eight people as part of a surge of attacks by militants around the country, officials said Tuesday.

A Taliban spokesman, Zabiullah Mujahid, said the group carried out the late Monday assault and described the victims as spies for the U.S. military.

Attacks by the Taliban and its Islamic State rivals have been rising in Afghanistan as the United States completes plans to send several thousand more troops to bolster Afghan forces and the 8,500 U.S. troops already in the country.

“We cannot rule out anything — enemy attack, local or tribal hostility,” provincial Police Chief Mohammad Zaman Mamozai said by phone in reference to the attack on the Bagram guards. He said two other occupants of the vehicle suffered bullet wounds.

Meanwhile, violence flared in a series of attacks around the country and in the capital, Kabul, between security forces and protesters who had occupied a busy avenue for the past three weeks.

Videos of the Kabul clashes posted on social media showed the body of a man who protesters said was shot by police late Monday. A number of wounded men were also shown lying on the street.

A protest organizer, Ramish Noori, said several people were arrested.

Afghanistan’s chief executive, Abdullah Abdullah, confirmed on live TV that one protester was killed and six others wounded. He called for an investigation into the clash.

“Last night’s event in Kabul has shocked all of us,” he said.

Protesters have expressed frustration with U.S.-backed President Ashraf Ghani and his inability to stem the rising tide of violence, including a devastating truck bomb in Kabul that killed 150 people on May 31.

After police opened fire on an anti-government protest three weeks ago, protesters set up tents and vowed to remain until the president resigned.

Afghanistan’s Independent Human Rights Commission denounced the police response as “illegal” and said it raised concerns about “intimidation and fear.”

“Tonight’s move against the protesters . . . is not acceptable, and the government is responsible for any legal and humanitarian violation,” Fawzia Koofi, a lawmaker, said in a message.

Omar Ahmad Parwani, an organizer of the protest, called for massive anti-government marches and blockades of key roads in the capital.

While frustration with Ghani’s government has been high among Afghans because of the poor economy, corruption and the rising number of attacks, the sit-in never attracted more than a few dozen protesters at a time, and many residents complained that it worsened the city’s already congested traffic.

Police have opened fire on protesters twice now, which might discourage more from taking to the streets.

In Helmand province, a Taliban stronghold in southwestern Afghanistan, four police officers were killed when their car hit a roadside bomb Tuesday, Omar Zwak, a spokesman for the provincial governor, told the Associated Press.

In eastern Nangahar province, a judge was killed when a bomb destroyed his vehicle, according to Attuhullah Khogyani, a spokesman for the provincial governor, the AP reported. The Islamic State claimed responsibility in a statement distributed by its media arm, the Amaq News Agency.