Beirut: Tens of thousands of Syrians carrying olive branches and shouting for the downfall of President Bashar Assad's regime on Friday streamed into a flashpoint city where the US Ambassador travelled to show his solidarity with protesters, witnesses said.

Two witnesses told a news agency that crowds were swelling in Hama, a central city that has become a focal point of the uprising and has drawn the largest crowds since the revolt began nearly four months ago.

People are chanting 'We only kneel to God', one of the witnesses told the news agency by telephone, as the sound of the crowd was heard in the background. He asked for anonymity, fearing for his personal safety.

Hama poses a potential dilemma for the Syrian regime because of its place as a symbol of opposition to the rule of the Assad family. In 1982, the late Hafez Assad ordered troops to crush a rebellion by Islamist forces, killing between 10,000 and 25,000 people, rights activists say.

A major offensive could make the city a fresh rallying cry for the opposition, but Assad's regime also does not want a repeat of last Friday's stunning rally, when an estimated 300,000 people protested.

US Ambassador Robert Ford's trip to Hama on Thursday drew condemnation from the Syrian Government, which said the visit was unauthorized and a clear sign that Washington was inciting unrest in the Arab nation.

Relations between the US and Syria are chronically strained over Assad's close ties with Iran.

"The presence of the US Ambassador in Hama without obtaining prior permission from the Foreign Ministry as stipulated by instructions distributed repeatedly to all the embassies is clear evidence of the US involvement in the ongoing events in Syria," a news agency reported, citing an unnamed "official source" at the Foreign Ministry.

The US is trying to "aggravate the situations which destabilize Syria," the statement said.

In Washington, State Department spokeswoman Victoria Nuland said Ford "spent the day expressing our deep support for the right of the Syrian people to assemble peacefully and to express themselves."

Ford reached the city after passing checkpoints run by the military and Hama residents. Nuland said he met nervous residents and saw many shops closed because of a protest-linked strike. He also visited a hospital treating the wounded.