Syrian security forces shot dead at least 26 protesters on Friday as thousands rallied on a "Day of Defiance" against the Syrian regime, a rights group said, with authorities accusing "terrorists" of killing 10 security personnel.
The violence came as the European Union agreed to impose sanctions on 13 Syrian officials involved in the regime's brutal crackdown on protests and will meet Monday to discuss whether to target Syrian President Bashar al-Assad as well, diplomats said.
The United States warned it would take "additional steps" against Syria if it continues its crackdown, a week after imposing tough sanctions on the Arab nation.
The Syrian rights group, Insan, put the day's toll at 26 killed but said it expected this to rise.
"Death toll for today is 26," Insan said in a statement received by AFP in Nicosia. The toll included 16 protesters killed in the central city of Homs, six in Hama, north of Damascus, and two in Jableh on the Mediterranean.
It did not specify where the other two deaths occurred.
"We have a list of 27 more names. We are verifying at the moment," the brief statement said.
Rights activist Najati Tayara, speaking to AFP from Homs, said security forces opened fire to disperse protests after weekly Muslim prayers in the central city, killing eight people and wounding several, some seriously.
Tanks entered the industrial city earlier and also rolled into other protest centres across Syria.
"Dozens of people were arrested in several districts of the city during the night between Thursday and Friday," Tayara said.
Using loudspeakers, security forces ordered protesters to voluntarily go to police stations "to avoid being arrested and punished" and ordered businesses to close and residents to stay indoors, activists said.
The military announced that 10 soldiers and policemen were killed in Homs by "armed terrorist groups", and said the bodies of some of them were mutilated by the assailants.
Prominent dissident and former MP Riad Seif, 64, was arrested in Damascus, the London-based Observatory reported. Amnesty International said he was seized while leaving a mosque.
Seif had been imprisoned twice since 2001, serving in total more than seven years in jail for seeking reforms and constitutional changes in autocratic Syria.
According to Amnesty "human rights activists involved in pro-reform protest in Syria have been forced into hiding after receiving threats from Syrian authorities."
Those identified by Amnesty include human rights lawyer Haytham al-Maleh, veteran dissident Walid al-Bunni and Suheir al-Atassi, who had been arrested when the protest movement erupted in mid-March and released in early April.
"The United States believes that Syria's deplorable actions toward its people warrant a strong international response," the White House said in one of its strongest statements yet since the outbreak of unrest there.
It warned that unless Assad's government halted its repression of peaceful pro-democracy protests, "the United States and its international partners will take additional steps to make clear our strong opposition to the Syrian government's treatment of its people."
The Syrian Revolution 2011, a Facebook group that has been a driving force of the protests, called for the "Day of Defiance" demonstrations, saying "Liberty is close."
Tens of thousands rallied across the country, including in the Kurdish areas of Amuda and Derbassiye in northeast Syria, in the coastal city of Banias, Kafar Nubbol in the north, Al-Bukamal near the border with Iraq and in Deir Ezzor in the east, activists said.
Thousands also swarmed Saqba, near Damascus, where 300 people were rounded up on Thursday.
The new troop deployment came as the army pursued its withdrawal Friday from the southern town of Daraa following a 10-day lockdown, during which the army said 600 were arrested.
On Thursday hundreds of troops withdrew from Daraa, the epicentre of Syria's protest movement which erupted March 15.
"Throughout the night, they withdrew from Daraa and this is continuing today. The troops' departure is gradual," General Riad Haddad, the military's political department chief, told AFP.
Human rights groups say that more than 600 people have been killed and 8,000 jailed or gone missing in the crackdown on protesters since demonstrations erupted in mid-March.
Friday's protests coincided with the Martyrs' Day public holiday in Syria, which Assad marked by laying a wreath at the Tomb of the Unknown Soldier in Damascus, according to official news agency SANA.
In a concession that failed to dampen the protest movement, Assad decreed an end to five decades of emergency rule on April 21, but his forces have continued to use deadly force and carry out arbitrary detentions.
The International Press Institute on Friday demanded that Syria release Al-Jazeera television journalist Dorothy Parvez, detained since she flew in to Damascus on April 29.
Washington said it was seeking diplomatic access to Parvez, who holds US, Canadian and Iranian citizenship.