Security forces killed 27 anti-government protesters around Syria on Friday, most of them in Damascus, amid indications that opposition to President Bashar al-Assad is hardening in the capital.

According to the Local Coordination Committees, a group that organizes and monitors protests, 22 people were killed in neighborhoods and suburbs of Damascus, the highest daily toll there since the nationwide uprising began four months ago. Activists said the protests in the capital were also the largest yet, pointing to what they say is a rising tide of anti-Assad sentiment in the heart of his government’s power base.

The violent response shows that the authorities “are 100 percent worried about Damascus,” said Rami Abdelrahman of the London-based Syrian Observatory for Human Rights, who estimated that 50,000 people took to the streets in and around the city.

Figures could not be independently confirmed because the government has restricted journalists’ access to Syria. But the reported size of the demonstrations remains small compared with those that have toppled rulers elsewhere.

The deadliest crackdown came in the suburb of Qaboun, where 14 demonstrators died after security forces fired on a what activists described as a demonstration of 25,000 people. Video posted on YouTube showed thousands marching through the streets chanting anti-government slogans and holding banners proclaiming “Out Bashar” and “Game Over Bashar” in English.

Qaboun has emerged as an opposition stronghold in recent weeks, and dissidents had been hoping to hold a conference there Saturday to draw up a strategy for ousting Assad in coordination with a parallel gathering of exiled opposition leaders in Istanbul. But after the killings Friday, organizers called off the Damascus gathering because of safety concerns.

Another video showed the biggest demonstration yet to be held in the central Damascus neighborhood of Midan, where authorities have repeatedly tried and failed to quell protests by blanketing the area with security forces.

Two eyewitnesses put the size of the crowd at several thousand and said scattered demonstrations continued for several hours despite efforts by the security forces to disperse protesters with batons and tear gas.

Elsewhere, far bigger demonstrations passed off without incident, in an indication that the government may be losing its grip in some parts of the country.

In the eastern border towns of Deir al-Zour and Bokamal, security forces made no attempt to prevent tens of thousands of people from taking to the streets. Large demonstrations also proceeded unhindered in the heart of the central city of Hama, which has effectively been taken over by government opponents since security forces withdrew to the city’s outskirts more than a month ago.

The scale of the protests in Damascus was perceived as significant because, until now, the capital has been considered a stronghold of government support. “Damascus has proved that it is changing,” said Wissam Tarif of the human rights group Insan.

Some activists say the protesters have been encouraged by signs that the international community is toughening its stance against Assad, following comments this week in which Secretary of State Hillary Rodham Clinton said Assad had “lost legitimacy.”

Abdelrahman also attributed the growing numbers attending protests in the capital to the end of the university year last week, which has freed thousands of students for the summer.

Opposition leaders had been hoping to boost momentum for the effort to oust Assad at Saturday’s dual-city conference, which aspired to unite traditional dissidents with the largely spontaneous and youthful protest movement that has sprung up on the streets of Syria.

With activists concluding that it is now too dangerous for them to meet in Damascus, however, the effort to forge a cohesive opposition movement remains in doubt.