Hundreds of thousands of people have attended an anti-government protest in the Syrian city of Hama, activists say.
The demonstrators signalled their rejection of the government's proposed national dialogue conference on Sunday.
At least 14 people were reportedly killed at other protests nationwide, including six in a Damascus suburb.
Earlier, the US and French ambassadors visited Hama to show their solidarity. The Syrian authorities called it an incitement to further aggression.
Tanks were deployed on the outskirts of Hama last weekend after the central city witnessed the largest protest since anti-government demonstrations began in March.
At least 22 people in Hama have since been shot dead by security forces.
"No-one can predict what is going to happen in the next few days," one resident told BBC Arabic. "Many families have left Hama for the neighbouring villages."
While there were protests throughout Syria, most attention was focused on Hama, where once again huge numbers turned out on the streets after Friday prayers, demanding the overthrow of President Bashar al-Assad and rejecting an offer of dialogue.
Rami Abdul Rahman of the Syrian Observatory for Human Rights told the AFP news agency as many as 450,000 people were taking part.
A witness told the Associated Press that many protesters were carrying olive branches and chanting: "We only kneel to God".
"There was no violence. As long as we have no security forces, we have no violence," another said.
One person told BBC Arabic that thousands of people from nearby villages had been prevented by security forces from entering Hama and joining the march.
The US and French ambassadors to Syria, Robert Ford and Eric Chevallier, travelled to the city on Thursday as acts of solidarity, but left before Friday's protests began, according to officials in Washington and Paris.
But the Syrian interior ministry said Mr Ford's visit, for which he did not seek the permission of the authorities in Damascus, was an act of incitement.
"The ministry wondered at the US ambassador's arrival in Hama contrary to the diplomatic norms and despite the roadblocks set up by the saboteurs to prevent citizens from reaching their jobs," state media quoted it as saying.
The ministry said Mr Ford had met "a number of the saboteurs and incited them to more violence and protest and to refuse dialogue".
It added that the ambassador, "under the cover of visiting some hospitals", had met other people in an attempt to encourage further violence and instability, to sabotage national dialogue, and to deepen discord and sedition among the Syrian people "who strongly reject and condemn such foreign instigation".
A video posted on YouTube on Friday showed a 4x4 vehicle driving through Hama's central Assi Square as young men chanted: "People want the downfall of the regime." People tossed flowers onto the vehicle, which the person making the video said was carrying Mr Ford.
US state department spokeswoman Victoria Nuland said it was "dismayed" by the criticism of Mr Ford's visit and dismissed the complaint that it was a provocation as "absolute rubbish".
"He witnessed average Syrians asking for change in their country," she said, adding that the authorities had been informed in advance.
Hama was the scene of a brutal crackdown in 1982 ordered by Hafez al-Assad, the president's late father, which left at least 10,000 dead.
At least 60 people were shot dead in Hama during protests on 3 June.
There were also mass demonstrations in other towns and cities across the country on Friday.
Activists said security forces shot dead six people in the western Damascus suburb of al-Dumair, two in the capital's central Midan district, and at least three others in the central city of Homs.
Three protesters were also killed in Maarat al-Numan, not far from the restive north-western town of Jisr al-Shughour, they added.
Overnight, three people were killed at a demonstration in Harasta, another of the capital's suburbs, activists said.
The death toll could not be independently confirmed as international journalists have been denied access to Syria.
Meanwhile, a spokesman for the Local Co-ordination Committees, which represents many protesters, insisted that they would send no-one to the government's national dialogue conference on Sunday and Monday.
"Dialogue with this regime is out of the question as we cannot talk to murderers," Mohammed al-Abdullah told al-Jazeera. "This is being declared by the rebels in the street."
"The city of Hama is being massacred and besieged, the Syrian people are being tortured, and therefore talk of any dialogue with the regime is a disgrace to the blood of the martyrs."
He added: "We do not believe that there are any members of the regime who believe in dialogue."
Officials have said there will be more than 200 delegates, including people from the opposition, the ruling Baath Party, and independents.
State media have reported that amendments to the constitution will be on the agenda at the meeting, including Article 8, which grants the Baath Party unique status as the "leader of state and society".
Participants will also reportedly examine proposed new laws on political parties, elections, local administration and the press.
Human rights activists say more than 1,400 civilians and 350 security forces personnel have been killed across the country since March.
The government has blamed "armed criminal gangs" for the unrest.