Syrian opposition urges Homs international protection
The main Syrian opposition group says Homs is a disaster area and is urging international protection for civilians.
The Syrian National Council said government forces had been shelling residential areas for days.
It says civilians are very short of food and medical supplies, and electricity and water have been cut.
Last week the Syrian government agreed to an Arab League deal to pull troops out of residential areas.
However, the BBC's Jonathan Head, in neighbouring Turkey, says this has not happened.
Opposition activists describe conditions in some parts of Homs, Syria's third largest city, as catastrophic.
They say government forces have cut electricity and water supplies.
Districts like Baba Amr, not far from the city centre, have been under almost constant bombardment by government forces for nearly a week, killing a total of more than 100 people, activists say.
They accuse government snipers of preventing medics from treating injured protesters.
The Syrian National Council statement also says that families, including women and children, are being prevented from leaving the areas of fighting and moving to safe areas.
It has accused the government of using heavy artillery, rocket launchers, and warplanes to bomb populated residential neighbourhoods.
The opposition claims are impossible to verify as the Syrian government has banned most foreign journalists from entering the country.
Reports from Homs say security forces on Monday entered Baba Amr which had earlier seen fierce fighting between government troops and Syrian army defectors.
Opposition activists say three people, including a child, have been killed so far on Monday in Homs.
On Sunday, 16 civilians were reported to have been killed by security forces while protesting in Homs.
The Syrian National Council statement urged the United Nations, the Organisation of Islamic Co-operation and the Arab League to act to stop what it called the "massacre" in Homs.
It called on the international community to send "Arab and international observers, instantly, to the city of Homs to oversee the situation on the ground".
The opposition is calling for Homs to be declared a "humanitarian disaster area", so that civilians can be given international protection, including evacuation from areas that are being shelled.
The statement also urges the UN and Arab League to call for an immediate ceasefire.
Opposition groups have also reported continuing protests, and civilian casualties, in several other parts of the country, including the suburbs of Damascus, Banias and Aleppo.
The Arab League is due to hold a meeting on Saturday to address the Syrian government's apparent failure to comply with a peace deal it accepted last week.
Instead, President Bashar al-Assad has offered amnesties for army deserters to turn themselves in and for civilians to hand over any weapons they have.
The United States has said it does not believe it would be safe for Syrian dissidents to surrender to the authorities, a statement which has prompted the Syrian government to accuse the US of backing what it called sedition and violence.