"They discovered a mass grave in the old part of town but authorities immediately cordoned off the area to prevent residents from recovering the bodies, some of which they promised would be handed over later," Qurabi said on the phone from Cairo.
He was unable to say how many bodies were found in the mass grave.
His account could not be independently verified as Syrian authorities have prevented journalists from traveling to cities and towns across the country to report on the unprecedented protests threatening the authoritarian rule of President Bashar al-Assad.
Qurabi said that 34 people had also been killed in the past five days in the towns of Jassem and Inkhil, near Daraa.
"I fear that dozens more casualties may be lying in nearby wheat fields and orchards because families have not been able to access the region which is encircled by security troops and snipers," he said.
In Tall Kalakh, the western town besieged by the army, a witness told AFP of corpses and wounded people left lying in the streets, with local residents unable to recover them because of shelling and heavy gunfire.
He said many corpses were also being kept in refrigerated trucks at the local hospital pending burial.
"I can see six tanks from where I am right now," said the man, who wished to remain anonymous.
"There are many more parked in front of the main bakery and the Othman ibn Affan mosque in the center of town," he added, as gunfire rattled in the background.
Another witness earlier said that at least 10 people had been killed on Sunday in Tall Kalakh with hundreds of local residents fleeing, many of them to nearby Lebanon.
Shelling and shooting was also reported in the nearby town of Arida, an activist told AFP.
Syrian television, meanwhile, reported that two soldiers had been killed and 11 injured by "gangsters" in Tall Kalakh at the weekend.
Security forces have sought to prevent the unrest from spreading across the country by systematically laying siege to towns and cities where anti-regime protests have been held.
More than 850 people, including women and children, have been killed in the unrest and at least 8,000 arrested, according to rights groups.
Syria has blamed the violence on "armed terrorist gangs" backed by Islamists and foreign agitators.
Hundreds of those arrested were released on Sunday after signing pledges not to take part in further protests, said Rami Abdul-Rahman, director of the Syrian Observatory for Human Rights.
"Several of them said they had been tortured," he said, adding that thousands of people remained jailed and more arrests were taking place.
The United States and the European Union have responded to the unrest in Syria by imposing sanctions on members of Assad's inner circle but stopped short of targeting him personally.
There are fears that should Assad's regime fall, that would have serious ramifications for the region and could lead to civil war.
Washington on Monday accused Syria of stoking protests in the Golan Heights, where deadly clashes took place at the weekend, as a "distraction" from its repression of anti-government protests and warned "such behaviour is unacceptable."
"It seems apparent to us that that is an effort to distract attention from the legitimate expressions of protest by the Syrian people and from the harsh crackdown that the Syrian government has perpetrated against its own people," said White House spokesman Jay Carney.
Fourteen people were killed Sunday and hundreds injured in Syria, Lebanon and the Palestinian territories after Israeli troops fired on thousands of protesters who sought to breach its northern border.
The protests were to mark the anniversary of Israel's founding in 1948, an event known in Arabic as the "nakba" or "catastrophe."