Syrian government forces killed 21 people around the country on Friday, activists said, with at least nine people killed by tank shelling in opposition districts of the central city of Homs.
The resumption of heavy shelling comes after a few days of relative calm during which U.N. humanitarian chief Valerie Amos visited Homs and said part of the city had been completely destroyed.
The Baba Amr district was besieged and shelled for a month before rebels opposed to president Bashar al-Assad fled and Syrian troops moved in.
Thirty tanks entered my neighbourhood at seven this morning and they are using their cannons to fire on houses, said Karam Abu Rabea, a resident of the Karm al-Zeitoun district in Homs.
There is gunfire and they are using rocket-propelled grenades, he said, adding that the streets had emptied and people were taking refuge in their homes.
In other parts of the country, Assad's opponents took to the streets to demonstrate after Friday Muslim prayers.
An activist in the Khalidiya district of Homs, also a centre of opposition to Assad's rule, said he had heard mortars landing in his neighbourhood since the early hours of the morning and the distant buzz of a spotter plane.
The grassroots Local Coordination Committees said nine people were killed in the city of Homs, three in Homs province, two in Idlib and two in Damascus.
The UK-based Syrian Observatory for Human Rights added that security forces had killed two people in house-to-house raids near the Turkish border and three were killed in Hama city.
Humanitarian chief Amos said on Thursday she had asked the Syrian government to allow access for humanitarian aid to victims of the turmoil.
U.N.-Arab League envoy Kofi Annan is due to arrive in Damascus on Saturday to try to calm a year-old conflict which threatens to turn into civil war.
(Additional reporting by Khaled Yacoub Oweis in Amman; editing by Andrew Roche)