In the worst violence since the Syrian uprising began in March, at least 136 people are reported to have been killed. In Hama, up to 95 people are dead after tanks and soldiers stormed the city.
The army has been besieging the city of Hama, which has seen some of the biggest demonstrations against President Assad, for nearly a month.
One doctor told the Reuters news agency: “Tanks are attacking from four directions. They are firing their heavy machine guns randomly and overrunning makeshift roadblocks erected by the inhabitants.”
Another resident said bodies were lying uncollected in the streets, and predicted the death toll would rise.
Tank shells were falling at the rate of four a minute in the north of the city, according to residents, and water and electricity supplies to the main neighbourhoods have been cut off.
The crackdown comes on the eve of Ramadan, which begins on 1 August.
In the Syrian capital, Damascus, troops arrested 100 people, while amateur video footage from the eastern city of Deir ez-Zor also shows invading tanks.
And in southern Syria, security forces reportedly killed six civilians when they stormed houses in the town of al-Hirak.
The UK Foreign Secretary, William Hague, said he was “appalled” by the reports from Hama. “Such action against civilians who have been protesting peacefully in large numbers in the city for a number of weeks has no justification,” he said.
“President Bashar is mistaken if he believes that oppression and military force will end the crisis in his country. He should stop this assault on his own people now.”
Meanwhile, US President Barack Obama has accused the Syrians of using torture, corruption and terror.
Channel 4 News reported on 8 July that tanks had been deployed around the outskirts of Hama following protests involving an estimated 500,000 people the previous weekend.
The previous day, 7 July, the US ambassador to Syria, Robert Ford, travelled to Hama in a gesture of solidarity with protesters there.
The activist group Avaaz said in a report last week that 1,634 people had been killed by security forces since the crackdown began. 2,918 had disappeared, and another 26,000 had been arrested – many of whom were beaten and tortured.
The Syrian leadership blames “armed terrorist groups” for most of the killings during the uprising in the country, saying that more than 500 soldiers and security personnel have been killed.
Hama, which is situated 100 miles north of the Syrian capital, Damascus, has a tradition of anti-government resistance.
In 1982 the city was the scene of a massacre when President Assad’s father, the late president Hafez al-Assad, sent in troops to crush an Islamist-led uprising. An estimated 30,000 people were killed.
President Basahr al-Assad is trying to end an uprising against his 11-year rule that broke out in March, inspired by the “Arab spring” revolutions in Tunisia and Egypt.