Rights group says 21 people rounded up by security forces day after witnesses say several protesters killed.
Syrian security forces have arrested more than 20 people, a rights group said, a day after at least four deaths were reported as thousands marched in pro-reform protests.
The Syrian Observatory for Human Rights named 21 people who had been rounded up on Saturday in the southern city of Daraa and in Homs to the north of the capital.
"It is assumed their arrests are as a result of the last protests," the group said in a statement.
"[The group] demands that the Syrian authorities release all detainees of opinion and conscience and to stop the practice of arbitrary arrests against political opposition and civil and human rights activists."
A day earlier, thousands took to the streets in major cities after Friday prayers, defying security forces who fired tear gas and live ammunition and used batons to try and disperse protesters who have dismissed a limited reform gesture by Bashar al-Assad, in power for the last 11 years.
Witnesses said security forces killed at least four protesters in the Damascus suburb of Douma on Friday.
The authorities have denied that the security forces were responsible for the deaths, blaming them on an "armed group" which opened fire from rooftops in the town on demonstrators and police alike.
They acknowledged that there were an unspecified number of deaths and said there were dozens of wounded, some of them police.
State television charged that "some of the demonstrators had daubed their clothes with red dye to make foreign reporters believe that they had been injured".
The total number of deaths since demonstrations began in Daraa on March 18 is unclear but activists say the toll is at least 60.
UN chief Ban Ki-moon is "deeply concerned" about the situation, a statement from his office said.
"The secretary-general is deeply concerned about the situation in Syria, where more civilian deaths have been reported during the latest popular demonstrations," it said. "He deplores the use of violence against peaceful
demonstrators and calls for it to cease immediately."
In a much-anticipated speech on Wednesday, his first since the protests erupted, president Assad failed to lift almost 50 years of emergency rule.
He later ordered the formation of a panel that will draft anti-terrorism legislation to replace emergency law, a
move critics have dismissed, saying they expected the new legislation would give the state much of the same powers.
Ending emergency law has been a central demand of protesters, who also want political prisoners freed, and to know the fate of tens of thousands who disappeared in the 1980s.
Assad also ordered an investigation into protest deaths in Daraa and Latakia, and formed a panel to "solve the problem of the 1962 census" in the eastern region of Hassake. Following the controversial census, about 150,000 Kurds living in Syria were denied nationality.
The developments come amid severe restrictions to media operations in Syria. The Damascus correspondent of the Reuters news agency was expelled last week. One foreign journalist was released by authorities on Friday, three days after he had been detained, while a Syrian Reuters photographer remains missing since Monday. Two other foreign Reuters journalists were also expelled.