Britain and France announced Wednesday that they would shutter their embassies in Yemen, a day after the United States closed its mission and withdrew its diplomats from the strife-torn Arabian Peninsula nation.

The departure of large numbers of diplomats highlights Yemen’s growing isolation amid a crisis that threatens to trigger full-scale civil war and create a security vacuum for al-Qaeda militants to plot attacks.

The French Embassy in Sanaa said Wednesday that it would suspend operations, while Britain announced that it closed its embassy in the capital and withdrew its diplomats from the country. A day earlier, the United States announced the closure of its Sanaa embassy and pulled out its staff, while a military official confirmed that the facility’s Marine guards also withdrew.

“The security situation in Yemen has continued to deteriorate over recent days,” Tobias Ellwood, a minister for the Middle East in Britain’s Foreign Office, said in a statement.

Meanwhile, tens of thousands of people rallied across the country Wednesday in demonstrations against the Houthi rebels, followers of the Zaydi sect of Shiite Islam. Yemen’s political crisis worsened after the Houthi insurgents effectively toppled the country’s U.S.-backed government last month and dissolved parliament last week.

The unrest also has raised questions over the continued cooperation of Yemeni authorities with a U.S. drone program that targets al-Qaeda’s branch in Yemen, known as al-Qaeda in the Arabian Peninsula (AQAP).

[Chart: Yemen’s chaos, explained]

Houthi officials confirmed Wednesday that the group’s fighters seized about 20 vehicles that were abandoned Tuesday at the airport by departing U.S. diplomats, including the ambassador.

In Washington, the State Department said the diplomats and other embassy staff were evacuated on private jets provided by Oman and flew to the Omani capital, Muscat, before traveling Wednesday to Washington. Department spokeswoman Jen Psaki emphasized that operations were “suspended” rather than terminated at the embassy, which she said would reopen when the security situation permits.

Psaki said the seizure of the vehicles was “unacceptable” and that “we are requesting their return.” Houthi leaders “have made public statements about how they have no desire to go after our interests, our materials,” she said. “We expect them to abide by their own statements.”

Opponents of the Houthis gathered in large numbers in the city of Taiz, where witnesses reported that thousands of people chanted slogans against the rebel militia and hoisted placards denouncing its takeover as a coup and illegitimate.

“We are doing this because we oppose the coup and we want a return to a state that is run by civil institutions, not a militia,” said Ibrahim al-Jabri, an activist from Taiz who participated in the protests.

Residents of Taiz have been especially vocal in their opposition to the Houthis, who have advanced far beyond the capital and threaten to spark a wider conflagration.

Jabri said there was rising concern that the Houthis’ advances could spark sectarian friction with the country’s majority Sunnis. “If they invade Taiz, we will be forced to fight, and it will be a catastrophe for us all,” he said.

Wednesday marked the fourth anniversary of the beginning of Yemen’s “Arab Spring” uprising that ousted the country’s former president, Ali Abdullah Saleh.

Few of the protesters who participated in that uprising turned out in the capital. Instead, it was the Houthis who took to Sanaa’s streets in large numbers Wednesday. Thousands of them gathered downtown, waving placards with the group’s provocative slogan: “Death to America, death to Israel, damnation to the Jews.”

Many rallied in Change Square, the epicenter of the 2011 uprising, which was propelled by student demonstrators. Houthi fighters erected additional barricades that effectively put parts of central Sanaa on lockdown, attempting to minimize protests against them.

“We’re here to reject foreign interference!” said Majd al-Din, 24, who spoke while participating in the pro-Houthi march in downtown Sanaa with an AK-47 assault rifle slung over his shoulder. In apparent solidarity with the protest, camouflage-painted military helicopters buzzed overhead.

Din was a referring to a speech Tuesday by the rebel group’s leader, Abdulmalik al-Houthi, in which he criticized the withdrawal of foreign diplomats and warned against outside intervention in Yemeni affairs.

The U.S. Embassy staff left the country in a hurry; unconfirmed reports said Marine guards left caches of weapons behind.

Abdulmalek al-Ajri, a member of the Houthis’ political bureau, said the seized vehicles would be returned to local staff at the U.S. Embassy on Wednesday evening, with a U.N. official observing the handover.

“Tonight we would deliver the cars to the person running security at the embassy, and a United Nations observer will be present,” he said.

Ajri said the U.S. Embassy was being guarded by Yemeni security forces, which have fallen under the Houthis’ control. The security forces have not entered the embassy compound, which is still being managed by the facility’s local Yemeni staff, he said.

The area around the U.S. Embassy was largely empty Wednesday. The street leading to the facility was still blocked off by concrete barriers and guarded by Yemeni security forces wearing the usual fatigues and carrying automatic weapons.

“There aren’t any Americans left at the embassy,” Ajri said by telephone.

“Inside the embassy, the local employees of the embassy and the existing security staff are still in there. There are no Ansar Allah members inside,” he said, using another name for the Houthi militia.

Ajri said he did not know how many embassy vehicles the group had seized at the airport. He claimed that a fight broke out over the vehicles between local embassy staffers, forcing Houthi fighters to intervene and seize them.

Karen DeYoung in Washington contributed to this report.