Syrian government forces killed 27 people and wounded some 340 on Tuesday when they unleashed a heavy artillery barrage on a rebel-held district of the city of Homs, activists said.
The bombardment rained down as International Committee of the Red Cross officials tried to negotiate a pause in the fighting to allow them to bring aid to civilians suffering in horrendous conditions after 18 days of attacks on Homs.
In Damascus, security forces opened fire on demonstrators overnight, wounding at least four, activists said, in the latest sign that the 11-month-old uprising against President Bashar Al-Assad is taking a grip on the capital as well as the provinces.
Western powers and the Arab League prepared for a meeting of the "Friends of Syria" contact group in Tunisia on Friday to pressure Al-Assad to step down, while Russia and China backed Al-Assad's own programme for reform.
Russia said it would not attend the meeting because the Syrian government would not be represented. The Russian Foreign Ministry suggested the United Nations Security Council should send a special humanitarian envoy to Syria.
Activists said government forces had launched the artillery attack on Homs after rebel fighters holding the opposition Baba Amro district blocked troops from entering.
"Several shells are falling each minute," activist Nader Al-Husseini told Reuters from the district, adding that two children had been among the victims.
Another activist said 27 bodies had been found, many dug out from under the rubble. "Others are still buried. Today the shelling is very fierce," he said.
The London-based Syrian Network for Human Rights said at least 250 shells and rockets had hit Baba Amro since the morning and Syrian air force planes were flying reconnaissance missions.
Activists said government forces backed by armour and under the control of Alawite officers, from the same minority sect as Al-Assad, had been closing in on Baba Amro, a Sunni Muslim neighbourhood, since the offensive on Homs began on 3 February.
Tanks are deployed in the Inshaat district next to Baba Amro, opposition sources said, and the Syrian Observatory for Human Rights said a convoy of more than 50 armoured vehicles was seen heading from Damascus towards Homs on Tuesday.
Homs, a city of one million people straddling the Damascus-Aleppo highway, has been at the heart of the uprising against Al-Assad's 11-year rule.
Dangerous conditions and government curbs on access make it hard for media to verify activist reports, but international rights and aid organisations confirm a bloody and desperate standoff in which several hundred people have been killed.
Al-Assad says the revolt, until recently limited mainly to the provinces, is the work of foreign-backed terrorists.
But crowds rallied in the capital Damascus on Monday night and at least four people were wounded when security forces opened fire, activists said.
"There were hundreds of demonstrators at the main square of Hajar Al-Aswad, and suddenly buses of security police and shabbiha [pro-Assad militia] turned up and started firing into the crowd," activist Abu Abdallah told Reuters by telephone.
Footage posted on YouTube, purportedly taken before the shooting, showed a crowd marching in Hajar Al-Aswad carrying placards in support of besieged Homs and singing "Eyes are shedding tears for the martyrs among Syria's youth."
Elsewhere, an activist group in Kfar Tkharim near the Turkish border said rebel fighters had killed five soldiers and captured two in an ambush on a government column.
An activist in Al-Qusair, about 32 kilometres (20 miles) southwest of Homs and close to the Lebanese border, said five people were killed and eight wounded when the northern part of the town came under heavy fire from army mortars and T-72 tanks.
"People in that area are hiding in their homes; they can't leave. Others are resisting. Those who are farther away are fleeing the town. Some people are so scared they're trying to leave anyway even if they are close to the fire," Abu Ansa told Reuters by telephone.
Activists in the western city of Hama said troops, police and militias had set up dozens of roadblocks, cutting neighbourhoods off from each other.
Ahmad Ramadan, a leader of the opposition Syrian National Council, said Al-Assad loyalists killed his brother Mahmoud when they riddled his car with gunfire in his home city of Aleppo.
"The regime has been accusing Mahmoud of sending food and medicine to Homs and he was receiving daily threats. He was hit in the head and neck and died immediately," Ramadan told Al Jazeera Arabic news channel.