Syrian security forces have stormed the central city of Hama, reportedly killing more than a dozen anti-government demonstrators.
The fourth-largest city, which has become a flashpoint in the uprising against the Syrian regime, was inundated with armed plain clothes men backed up military troops, Al Jazeera reported.
Witnesses said they shot people with live rounds and tanks surrounded the city's entrances.
Tens of thousands of anti-government protestors have taken to the streets in the past few days, it said.
"It really is a scary situation in the area," said one witness broadcast on Al Jazeera.
President Bashar Assad's authoritarian regime is facing the most serious challenge to his family's four decades of rule in Syria.
The National Organization for Human Rights reported that Syrian troops have killed at least 22 people in Hama, Australia's ABC News reported.
"At least 22 people were killed in Hama and more than 80 wounded, some of them seriously," spokesman Ammar Qurabi said.
"The wounded are being treated in two hospitals in Hama," he said in a statement, adding that troops had entered the Al-Hurani hospital.
"A large number of Hama residents have fled either to the nearby town of Al-Salamiya or towards Damascus", ABC reported him as saying.
The latest violent crackdown since the uprising began mid-March comes as Amnesty International announced that security forces may have committed crimes against humanity during a deadly siege in May, The Associated Press reports.
During the siege in May, there were witness accounts of deaths in custody, torture and arbitrary detention, according to Amnesty, which has called on the UN Security Council to refer the situation in Syria to the International Criminal Court.
Activists say security forces have killed more than 1400 people since March. The regime disputes this toll, blaming "armed thugs" and foreign conspirators, AP reports.
The Amnesty report, issued on Wednesday, said the attacks "appear to be part of a widespread, as well as systematic, attack against the civilian population," which would constitute crimes against humanity.
Amnesty quoted witnesses as saying Syrian forces fired on fleeing families and ambulances carrying the wounded; one witness said soldiers stabbed lit cigarettes on the backs of detainees' necks.
At least nine people died in custody, witnesses told Amnesty. Eight of them were shot at and wounded as they were ordered out of a house, and were then taken away by soldiers.
Amnesty cited interviews carried out in Lebanon and by phone with more than 50 people.
The rights group, along with most foreign media, has not been allowed to enter Syria.
At the time of the Talkalakh operation, The Associated Press interviewed residents who told of a catastrophic scene in the town of about 70,000, including sectarian killings, gunmen carrying out execution-style slayings and the stench of decomposing bodies in the streets.
Some activists have said the death toll from the May siege was as high as 36 people.
"The accounts we have heard from witnesses to events in (Talkalakh) paint a deeply disturbing picture of systematic, targeted abuses to crush dissent," said Philip Luther, Amnesty International's Middle East and North Africa Deputy Director.
Talkalakh is just across the border from Lebanon.