The shelling of the neighbourhood of Baba Amr began at first light, the Syrian Observatory for Human Rights said, as sporadic explosions were heard in other areas of the central flashpoint city.
Four civilians were killed in Baba Amr and other Homs neighbourhoods, as security forces also opened fire on the neighbourhoods of Khaldiyeh and Hamidiyeh, in the centre of the city, the Observatory said.
Opposition group Local Coordination Committees said security forces fired mortars at Khaldiyeh.
Meanwhile, five people were shot dead as security forces clashed with defected troops in the town of Ezaz, the Observatory said.
Police also opened fire on a demonstration of some 4,000 people who rallied in the neighbourhood of Sayef al-Dawla, in Aleppo, the Observatory said.
At least 53 people were killed yesterday, as forces loyal to embattled President Bashar al-Assad bombarded Baba Amr, attacked villages and opened fire on demonstrators, as tens of thousands rallied across the country.
Twenty-two people were killed in Baba Amr itself, the Britain-based Observatory said.
Red Cross and Red Crescent ambulances had entered the besieged district of Baba Amr yesterday and evacuated seven wounded Syrians, as well as 20 women and children.
But the ambulances did not evacuate two wounded Western journalists and the bodies of two others, according to Saleh Dabbakeh, the Damascus spokesman for the International Committee of the Red Cross.
More than 7,600 people have been killed in violence across Syria since anti-regime protests erupted in March 2011.