More than 500,000 Syrians flooded through the city of Hama on Friday, according to activists, in what they claim was the single biggest protest yet against the embattled government of President Bashar al-Assad.
The opposition reported 13 protesters killed, including five deaths in the central city of Homs, two in the capital's commercial neighbourhood Midan and six in the Dumair area, east of Damascus.
Syrian state-run TV said the deaths in Damascus and Homs were caused by snipers from "armed gangs".
An activist told Al Jazeera that Hama, where marchers were seen carrying olive branches, had become a "tangible example of resistance to injustice" in Syria.
Hundreds of thousands also protested last Friday in Hama, prompting mass arrests and reports of several deaths when Syrian security forces stormed the city, Syria's third largest, and the surrounding area.
"Hama, with all the support it is receiving from all over the country, is becoming a role model for peaceful demonstrations and we are protesting here for all of Syria," the local activist said.
Friday's protests followed a visit to Hama by Robert Ford, the US ambassador to Syria, who toured the city on Thursday to show solidarity with residents, the US State Department said.
Ford reached the city after passing checkpoints run by the military and Hama residents.
A US official said Ford left Hama on Friday afternoon to avoid becoming a distraction during the weekly demonstrations.
Diplomats said on Friday that French ambassador Eric Chevallier was also in Hama to show support.
Damascus accused Washington of "interfering" in its affairs.
"The presence of the US ambassador in Hama without previous permission is obvious proof of a clear evidence of the United States' involvement in current events in Syria and its attempt to incite an escalation in the situation, which disturbs Syria's security and stability," the Syrian foreign ministry said in a statement.
In response, the US state department said: "The fundamental intention was to make absolutely clear with his physical presence that we stand with those Syrians who are expressing their right to speak for change."
In another development, Washington summoned the Syrian ambassador for talks this week after reports that embassy staff had filmed US protests against the crackdown in Syria.
Ambassador Imad Mustapha was called in to meet with top State Department officials "to express a number of our concerns with the reported actions of certain Syrian embassy staff in the United States".
Mustapha met with Assistant Secretary of State for Diplomatic Security Eric Boswell on Wednesday, according to a State Department statement.
An estimated 1,000 Syrians have fled Hama in fear of another military crackdown on protests calling for Assad to quit and an end to the Baath Party's decades-long grip on power, a rights group said.
The London-based Syrian Observatory for Human Rights said the residents had evacuated to Salamiyah, a town 30km from Hama, on Thursday. The rights group claimed that security forces have killed at least 23 civilians there and conducted mass arrests since Tuesday.
Ammar Qurabi, head of the National Organisation for Human Rights, said on Wednesday that an influx of troops following the massive Friday protest had brought a dramatic escalation of "killings and arrests in the city".
In-depth coverage of escalating violence across Syria
But Al-Watan, a state-run newspaper, said on Thursday that the situation in Hama was calm and the barricades erected in the streets by protesters to keep security forces out had been dismantled.
The newspaper said authorities had told demonstrators to avoid any confrontations and clear the streets so residents could go to work.
They also told protesters to avoid a "last resort" military operation, the paper said.
Hama has been a symbolic city of opposition since the 1982 crackdown on a revolt by the banned Muslim Brotherhood against then-president Hafez al-Assad, father of the current leader.
At least 20,000 people are believed to have been killed in the crackdown.
There has also been violence in the city of Hasrata just outside Damascus where three people have been killed and nine injured by security forces, sources told Al Jazeera.
Government troops surrounded the Hassan mosque on Thursday and fired at people coming out after prayers, the source said.
Police also fired tear gas into the local hospital, according to reports.