BEIRUT — Angry supporters of President Bashar Assad’s regime hurled tomatoes and eggs at the U.S. ambassador to Syria on Thursday as he entered the office of a leading opposition figure and then tried to break into the building, trapping him inside for three hours.

The Obama administration blamed the Syrian government for the attack in Damascus, saying it was part of an ongoing, orchestrated campaign to intimidate American diplomats in the country. Secretary of State Hillary Rodham Clinton condemned the attack as “wholly unjustified.”

Ambassador Robert Ford, an outspoken critic of Assad’s crackdown on the 6-month-old revolt against the regime, has angered Syrian authorities before by showing support for the uprising. The latest incident promises to raise tensions even further.

Ford came under attack by about 100 Assad supporters as he arrived for the meeting with Hassan Abdul-Azim, who heads the outlawed Arab Socialist Democratic Union party. Abdul-Azim is a strong critic of Assad’s regime and was briefly detained by authorities earlier this year.

Such incidents are usually not spontaneous in Syria, and Thursday’s attack came amid high tension between the two nations, as well as accusations by Damascus that Washington is inciting violence in the country.

The protesters were ready with eggs and tomatoes when the delegation arrived and threw them at the Americans as they entered the building.

In Washington, the State Department said a rowdy, violent mob tried to attack Ford and several American embassy workers in Damascus. Spokesman Mark Toner said Ford and his colleagues were unharmed and are now safe.

However, several heavily armored embassy vehicles sent to help extricate them from the situation were badly damaged with broken windows and dents when the same crowd hurled rocks, said White House and State Department officials.

“This inexcusable assault is clearly part of ongoing campaign of intimidation aimed at diplomats … who are raising questions about what is going on inside Syria,” Clinton said. “It reflects an intolerance on the part of the regime and its supporters.”