Syrian security forces have opened fire on anti-government protesters in cities across the country, leaving at least 28 people dead, activists say.
At least 16 people died in the capital, Damascus, where about 20,000 took part in protests, reports say.
The demonstrations appear to be among the largest since the anti-government uprising began in March.
The government has launched a "national dialogue", but many protesters want President Bashar al-Assad to quit.
Protests have regulary taken place following Friday prayers.
There were reports of mass demonstrations in several locations including the cities of Homs, Hama, Deraa, Deir al-Zour and Idlib and in the Damascus neighbourhood of Qabun.
The Syrian League for the Defence of Human Rights estimated about 20,000 people were protesting in Damascus.
Rami Abdel Rahman, of the Syrian Observatory for Human Rights, said more than one million people turned out in just two cities - Hama and Deir al-Zour.
"It's a major development and a message to the authorities that protests are getting bigger," he said.
Rights activists said at least 16 people had died in Damascus, three in Idlib, two in Deraa and three in Duma.
There were also reports of two deaths in Homs.
International journalists have been denied access to Syria and the figures cannot be independently verified.
Opposition activists told the BBC that protesters in Damascus were trying to evacuate several wounded people and to stop security forces from entering Qabun.
In Deraa, an activist told AP news agency: "All hell broke loose, the firing was intense."
The official Sana news agency reported that "armed men fired on security forces and citizens in the areas of Qabun and Rukn Eddin in Damascus".
US Secretary of State Hillary Clinton reiterated on Friday that President Assad had lost his legitimacy in the eyes of the Syrian people.
Speaking during a visit to Turkey, she said: "We, along with many others in the region and beyond, have said we strongly support a democratic transition," she said.
"The ultimate destiny of the Syrian regime and Syrian people lies with the people themselves."
President Assad is trying to crush a rebellion that human rights groups say has so far killed about 1,400 civilians and 350 security forces personnel.
The government blames the unrest on "armed criminal gangs" backed by a foreign conspiracy.
In an attempt to defuse the unrest, ministers recently held a two-day "national dialogue" between members of the ruling Baath party and its opponents.
However, many opposition leaders and protest organisers refused to attend.