Media playback is unsupported on your device Media caption Amateur footage of what is believed to be the city of Hama as Syrian forces attack

Syrian security forces have cracked down on anti-government protests across the country, killing 100 people in the city of Hama alone, reports say.

Witnesses said tanks moved into Hama at dawn, shelling civilians. Other towns also erupted in violence in one of the bloodiest days since protests began.

The government said troops had been sent in to Hama to remove barricades erected by the protesters.

US President Barack Obama said reports from Hama were horrifying.

"Once again, President [Bashar al-Assad] has shown that he is completely incapable and unwilling to respond to the legitimate grievances of the Syrian people," he said.

Mr Obama, who said he was appalled by the government's use of "violence and brutality against its own people", added that the US would continue to work to isolate Mr Assad's regime.

Analysis In over four months of protests, no-one can predict with any confidence what the outcome will be. The uprising won't go away but has yet to engulf the two biggest cities of Damascus and Aleppo. The protesters face a government that is talking about comprehensive reforms, but hitting back with ferocity. The Americans have not explicitly called for President Assad to go. The international community is not united on this in the way it was on Libya. So there is not going to be any outside intervention. It is up to the Syrians themselves, and at this stage, nobody can say how it will go.

By early evening, activists in Hama told the BBC that the city was quiet, and that the tanks had pulled out to the city's perimeters after failing to gain control of the centre.

With this latest military operation, the authorities are sending a clear message that they will not tolerate large-scale unrest ahead of the month of Ramadan, when protests are expected to grow, says the BBC's Lina Sinjab in Damascus.

But our correspondent says the people of Hama remain defiant, with some still out in streets shouting: "We will not be killed again," a reference to a massacre in 1982 when tens of thousands were killed.

Elsewhere in Syria, activists said about 30 people had been killed on Sunday amid widespread clashes:

Witnesses said security forces in the Damascus suburb of Harasta threw nail bombs into a crowd of protesters, injuring about 50 people

In the Damascus suburb of Muadhamiya, more than 100 people were arrested, rights groups said

Residents in the southern town of Hirak said four civilians have been killed and dozens more injured or detained

At least seven civilians were killed in the eastern provincial capital of Deir al-Zour, where tanks are patrolling the streets, according to activists

A powerful tribal leader, Nawaf al-Bashir, was detained by secret police in Damascus

The government said five soldiers, including a colonel, have been killed across the country

The recent protests - calling for widespread democratic reforms and political freedoms - show no sign of letting up despite a government crackdown that has brought international condemnation and sanctions.

Activists say more than 1,500 civilians and 350 security personnel have been killed across Syria since protests began in mid-March.

More than 12,600 people have been arrested and 3,000 others are reported missing.

Centre of protests

Hama has been in a state of revolt and virtually besieged for the past month. According to activists on the ground, troops and tanks began their assault at dawn, smashing through hundreds of barricades erected by locals to reach the centre of Hama.

"[Tanks] are firing their heavy machine-guns randomly and overrunning makeshift road blocks," a doctor in Hama told Reuters by phone, with machine-gun fire in the background.

Significance of Hama Hama - a bastion of dissidence - occupies a significant place in the history of modern Syria. In 1982, then-President Hafez al-Assad, father of Bashar, sent in troops to quell an uprising by the Sunni opposition Muslim Brotherhood. Tens of thousands were killed and the town flattened. Hama, with a population 800,000, has seen some of the biggest protests and worst violence in Syria's 2011 uprising. It was slow to join in, but has now become one of the main focuses of the revolt, and is largely out of government control.

Rights groups, residents and hospital officials in Hama told the BBC that 88 people had been killed in Sunday's operation.

Some residents said bodies were lying in the streets, and electricity and water supplies had been cut.

A Hama resident told the BBC World Service that the three main hospitals had run out of blood supplies after being overwhelmed by numbers of wounded people.

"They are treating people in the halls of the hospitals. A lot of injured people [have been] taken to homes and doctors are treating them there," he said.

The Syrian government defended its actions, saying in a statement on the state news agency Sana that armed groups had "set police stations on fire, vandalised public and private properties, set roadblocks and barricades and burned tyres at the entrance of the city".

"Army units are removing the barricades and roadblocks set by the armed groups at the entrance of the city."

Most foreign media is banned from the country, making it difficult to verify reports.