Tens of thousands of people rallied in the Syrian city of Hama, calling for the downfall of President Bashar al-Assad, as Western envoys visited the city to show support for protesters.

Activists said security forces shot dead 13 people elsewhere, including six in the town of Dumair near Damascus.

Hama, 200km north of Damascus, has seen some of the biggest demonstrations against Assad and was also the site of a brutal crackdown by his father nearly 30 years ago.

Assad deployed tanks outside the city this week.

Footage on the internet showed a huge crowd in Hama's Orontes Square, some carrying a Syrian flag tens of metres long and chanting ‘The people want the downfall of the regime’.

In a symbolic show of solidarity, US ambassador Robert Ford and French ambassador Eric Chevallier visited Hama to put pressure on Assad not to crush the protest.

Syria condemned Mr Ford's visit as incitement and proof that Washington was playing a role in 15 weeks of unrest, which have challenged Assad's grip on power.

The Interior Ministry said Mr Ford met ‘saboteurs and incited them to violence, protest and rejection of dialogue’, which authorities say will begin on Sunday.

The United States rejected the Syrian charge, saying the American envoy was welcomed with flowers and olive branches by peaceful civilians seeking political change.

‘When he got into the city the car was immediately surrounded by friendly protesters who were putting flowers on the windshields, they were putting olive branches on the car, they were chanting 'down with the regime,’ said State Department spokesman Victoria Nuland.

In a now familiar pattern of defiance, protesters emerged from mosques to protest in the capital, the southern city of Deraa where protests first erupted, the industrial city of Homs and other towns nationwide.

Activists said security forces shot dead six people in Dumair. Rights campaigner Ammar Qurabi said three people were killed in Maarat al-Numaan on the eastern edge of Idlib province, and two in the central Damascus district of Midan. Three people were killed in other incidents, he said.