An international human rights group called Sunday for a UN inquiry into Syria's crackdown on opposition protesters that has left more than 120 dead people in recent days.
The New York-based Human Rights Watch also urged the United States and European Union to impose sanctions against Syrian officials accused of using force against demonstrators challenging the authoritarian rule of President Bashar Assad.
Ammar Qurabi, head of the National Organization for Human Rights in Syria, said the arrests concentrated on the capital Damascus and suburbs as well as the central city of Homs, which has been a hotbed of demonstrations.
"These people are not being arrested in a legal way. They are being kidnapped," Qurabi said, claiming the plainclothes security agents did not have formal arrest warrants.
Qurabi did not have full figures for those detained, but said at least 20 people were arrested in Homs. A resident in the Damascus suburb of Douma, said at least five people were taken into custody
More than 300 people have been killed — including more than 120 on Friday and Saturday and eight on Sunday — since the uprising against Assad's regime began five weeks ago, according to rights groups. Friday was the deadliest day since the uprising began with 112 killed, rights groups said.
A coalition of 10 Syrian human rights groups said Sunday authorities have detained a leading activist, Daniel Saoud, who heads the Committees for the Defence of Democracy, Freedoms and Human Rights, one of the most prominent watchdog groups in Syria.
'Faced with the Syrian authorities shoot-to-kill strategy, the international community needs to impose sanctions on those ordering the shooting of protesters.'—Joe Stork of Human Rights Watch
Saoud, who was detained Saturday, lives in the coastal city of Banias, said a statement from the groups.
Starting Saturday night, Syrian security forces began detaining dozens of opposition activists and others in raids launched less than a week after Assad's regime abolished emergency laws used for decades to crush dissent.
"After Friday's carnage, it is no longer enough to condemn the violence," said Joe Stork, deputy Middle East director for Human Rights Watch. "Faced with the Syrian authorities shoot-to-kill strategy, the international community needs to impose sanctions on those ordering the shooting of protesters."
Earlier this month, UN Secretary-General Ban Ki-moon called Assad to say he was "greatly disturbed" by the reports of violence. Many Western leaders, including U.S. President Barack Obama, have condemned Syria's harsh tactics to quell dissident.
On Sunday, state-run news agency SANA said 286 police officers have been wounded since the uprising began. It did not give further details.
Many of the 13 killed Saturday were shot while on their way to bury loved ones from the attacks a day earlier, witnesses said.
Funeral procession times staggered
In the central city of Homs, where 19 people were killed Friday, a witness said security forces did not allow all funerals to go out at the same time.
"Security officials decide when the dead person will be buried in order to avoid large funerals," the witness said.
In the southern village of Izraa — where others were killed Friday — a witness said some men in the village chanted: "May God destroy Assad."
The witness accounts could not be independently confirmed because Syria has expelled journalists and restricted access to trouble spots. Witnesses spoke on condition of anonymity for fear of reprisals.
The protest movement has become the most serious threat to Assad's hold on power in one of the most rigidly controlled countries in the Middle East.
Assad has blamed most of the unrest on a "foreign conspiracy" and armed thugs trying to sow sectarian strife. Fears of sectarianism are strong in Syria with the dangers of fractured societies so apparent in neighbouring Iraq and Lebanon.
'People are afraid of going out, unless to do important things.'—an Iraqi Christian in Syria
But possible cracks could be emerging from within. Two legislators and a religious leader from the southern province of Daraa — where the uprising began — resigned Saturday in disgust over the killings.
Such internal rifts have added resonance since nearly all opposition figures have been either jailed or exiled during the 40-year dynasty of the Assad family.
Another walkout came Sunday when a member of the Daraa provincial council, Bassam al-Zamel, left his post to denounce the violence.
"It is our duty to resign in protest," al-Zamel told Al-Jazeera TV.
Travel warnings for foreigners, including Canadians
On Sunday, Britain's Foreign Office advised against all travel to Syria because of the violence. It added that "in light of the deteriorating security situation, British nationals in Syria who have no pressing need to remain should leave by commercial means."
Canada's Foreign Affairs Department on Saturday advised Canadians against travel to the country and said Canadians living in Syria should leave by commercial means while they're still available.
Many Iraqi Christians, who fled to Syria to escape violence in their homeland, did not attend Sunday Easter Masses because of the unrest.
Abu Fadi, an Iraqi Christian, said that he and his family had attended midnight Mass in a church in central Damascus for the past five years.
"This time, we and others families decided to stay home and even not to attend the morning mass due to the tense and unstable current situation in Syria. People are afraid of going out, unless to do important things," said Abu Fadi, who did not use his real name because of fears of reprisals from authorities.