The deaths came as the army moved in on two border towns in the east and west seeking to quell anti-regime protest.
"More than 30 civilians have been killed over the past 24 hours in Homs in clashes that broke out late on Saturday between the opposition and supporters of the regime," said Rami Abdel Rahman of the London-based Syrian Observatory for Human Rights.
|People demonstrate with Syrian national flags and candles in front of Lyon cityhall, centraleastern France, on July 16, 2011, in support for Syrian people.|
He said the clashes in the city 160 kilometres (100 miles) north of the capital came after three regime supporters kidnapped last week were killed and their dismembered bodies were returned to their relatives on Saturday.
"These clashes are a dangerous development that undermines the revolution and serves the interests of its enemies who want it to turn into a civil war," he added.
"The two sides started out beating each other with sticks, but then firearms were used."
Abdel Rahman said a large number of the dead were killed by gunmen lying in ambush, and that security forces did not intervene.
"Their duty is to maintain national security and protect citizens, not stand idly by when faced with clashes, as this can encourage even more violence," he charged.
A witness who spoke to AFP in Cyprus said the clashes were between Sunni Muslims and Alawites, Assad's sect, and that they occurred overnight in the Hadara and Al-Zahara districts of Homs.
Earlier Abdel Karim Rihawi, who heads the Syrian League for the Defence of Human Rights, reported the army moving in to Homs after clashes on Saturday.
The military on Sunday also set its sights on the town of Zabadani near the Lebanon border as it pressed ahead in its campaign to overcome the revolt, another rights activist said.
In the east, on the frontier with Iraq, security forces were also reported to be preparing to intervene in Al-Bukamal after one man was reported killed there on Saturday.
"Security forces today penetrated Zabadani," 50 kilometres (30 miles) northwest of Damascus, Rihawi said. "They searched houses and arrested more than 50 people."
Zabadani has seen several protests since anti-regime demonstrations erupted in mid-March.
Official media on Sunday reported an "explosive" situation in Al-Bukamal.
"The situation in Al-Bukamal is explosive, so the army is preparing to intervene," said the pro-government daily Al-Watan.
"The authorities fear an armed revolt in this border town where (insurgents) can easily find logistical and political support."
One civilian was killed in the area on Saturday when security forces opened fire to break up an anti-regime demonstration, the Syrian Observatory said.
However, the official SANA news agency spoke of "armed terrorist gangs who stormed a government building and seized the weapons stored there," adding that three security personnel were killed and two kidnapped in the attack.
Since the anti-regime protests began, Damascus has consistently blamed the violence on foreign interference and "armed groups" seeking to "sow chaos."
Activists say the protesters are peaceful and that the fierce government crackdown has left more than 1,400 civilians dead and thousands of others behind bars.
Al-Watan said the "situation was back to normal" in the central city of Hama, the epicentre of anti-government protests earlier this month.
"The efforts the new governor of Hama has made with civic leaders have borne fruit. The state of civil disobedience which lasted 13 days is over," according to Al-Watan.
Hama residents had raised barricades to prevent a military operation against the city, where memories of a 1982 crackdown against Islamists that left 20,000 people dead remain fresh.
Activists say security forces have killed least 25 civilians in the flashpoint city since July 5, when Damascus sent in tanks in response to an anti-regime demonstration that drew half a million people.
Meanwhile, security forces on Sunday rearrested Ali Abdullah, a key opposition figure, Rihawi's rights group said.
Abdullah was detained during a "vast campaign of arrests" focused on Qatana west of Damascus.
One of the 12 signatories of the 2005 "Damascus Declaration" which called for Syria to move towards democracy, Abdullah had been released under a general amnesty Assad declared in May.
The 61-year-old author, who had served a sentence of two and a half years for "undermining the image of the state" and "spreading false news," was originally set for release in June 2010.
But he remained in custody over comments he made in prison on Syrian-Lebanese relations and alleged electoral fraud in Iran.