Near a half million protesters jammed the streets of the flashpoint city of Hama in an anti-Syria government rally on Friday, as the U.S. ambassador traveled there to show American solidarity with protesters.

Rights activists told Western news media that crowds in Hama topped more than 450,000 – one of the largest gatherings since the uprising began four months ago.

Activists in Syria also say security forces have raided a Damascus suburb to quell an overnight protest against Syrian President Bashar al-Assad. Security forces reportedly fired bullets at civilians participating in the demonstration.

The Reuters news agency reported at least four dead in the siege and numerous injuries.

In Hama, the Associated Press reported that United States Ambassador Robert Ford left the city by midafternoon as crowds swelled.

The Syrian foreign ministry said Thursday that Ford's presence in Hama without prior permission constitutes “clear evidence” of a U.S. attempt to “increase tension and damage Syria's security and stability.”

In Washington, State Department spokeswoman Victoria Nuland said Ford met with a dozen people in a visit designed to show solidarity with protesters. She said Ford hoped to stay in the city until Friday and that the U.S. embassy had informed the Syrian government that an embassy team was travelling to the city.

Earlier, a Syrian activist monitoring developments in Hama told VOA in a telephone interview that dozens of families fled the city Thursday fearing a military crackdown. She said security forces have surrounded Hama with tanks, and that troops have detained more than 100 people.

The activist confirmed reports from rights groups that at least 25 people have been killed and more than 40 wounded in Hama in recent days. Troops had pushed through improvised barriers and roadblocks set up by Hama residents after massive anti-government protests last week.

United Nations Secretary-General Ban Ki-moon urged Syria to stop the deadly crackdown. Mr. Ban asked the Syrian leadership to grant access in the country to U.N. aid workers and a fact-finding mission.

Rights groups say Syrian security forces have killed at least 1,400 civilians since mid-March while trying to suppress the anti-government uprising. The Syrian government says terrorists and Islamist militants have killed hundreds of security personnel during the same period.