Syrian security forces opened fire on protesters Friday, killing at least six people as thousands joined demonstrations across the country calling for an end to President Bashar Assad's regime, witnesses and activists said.
Syrian authorities also detained Riad Seif, a leading opposition figure and former lawmaker who has been an outspoken critic of the regime during the seven-week uprising, according to the Syrian Observatory for Human Rights.
Five people were killed in the central city of Homs and one was killed in Hama, said a senior member of a human rights group that compiles death toll figures in Syria.
Like most activists and witnesses who spoke to The Associated Press, he asked that his name not be used for fear of reprisals by the government.
"We were chanting, peaceful, peaceful, and we didn't even throw a stone at the security forces," said a witness in Homs. "But they waited for us to reach the main square and then they opened fire on us."
From Hama, footage posted on YouTube showed protesters frantically trying to resuscitate a man lying on the ground with a bloodied face and shirt, while people shouted "God is great!"
The protesters turned out Friday despite a bloody crackdown on the uprising. More than 565 civilians and 100 soldiers have been killed since the revolt began in March, according to rights groups.
Rallies were held in major areas including the capital, Damascus, and its suburbs, the central city of Homs, Banias on the coast and Qamishli in the northeast.
"The people want to topple the regime!" protesters shouted, echoing the cries heard during the revolutions in Egypt and Tunisia.
Witnesses also reported some of the tightest security seen since the protests began in mid-March. In the Damascus suburb of Douma, scene of intense protests over recent weeks, security forces cordoned off the area to prevent anyone from entering or leaving.
A witness near Douma said he saw a train carrying about 15 army tanks heading north Thursday evening toward the central province of Homs, another site of recent violence.
Another activist in Damascus said hundreds of people marched in the central neighborhood of Midan. In Banias, witnesses said more than 5,000 people carrying olive branches and Syrian flags also were calling for regime change.
They were among several demonstrations and marches planned for Friday, the main day of protests in the Arab world, for what activists were calling a "Day of Defiance."
More than 565 civilians and 100 soldiers have been killed since an anti-regime uprising, inspired by revolts in Tunisia and Egypt, began in March, according to rights groups.
The activists said security forces set up checkpoints and closed some areas that experienced protests in recent weeks.
In the southern city of Daraa, where the army announced the end to an 11-day military operation Thursday, residents said troops were still in the streets, causing some would-be demonstrators to be wary of taking part in a planned protest Friday.
"There's a tank stationed at each corner in Daraa. There is no way people can hold a protest today," a resident said by telephone. "It means more killing. Daraa is taking a break. We don't want to see more killing or face tank guns."
The activists spoke on condition of anonymity for fear of government reprisals.
The International Committee of the Red Cross said a medical team reached Daraa on Thursday with trucks carrying humanitarian goods and medical supplies. The ICRC's head of delegation in Damascus, Marianne Gasser, said helping people in Daraa is a priority "because it is the city that has been hardest hit by the ongoing violence."
The ICRC had appealed to Syrian authorities earlier in the week to allow it to access to Daraa after being unable to reach the city previously while it was under siege by security forces.
Assad is determined to crush the revolt that has now become the gravest challenge to his family's 40-year dynasty. He has tried a combination of brute force, intimidation and promises of reform to quell the unrest, but his attempts have failed so far.
Security forces have repeatedly opened fire on protesters during rallies around the country in the past week and last Friday at least 65 people were killed, according to rights groups.
The mounting death toll -- and the siege in Daraa -- has only served to embolden protesters who are now demanding nothing less than the end of Assad's regime. There also has been growing international condemnation of the government's tactics.
Syria blames the unrest on a foreign conspiracy and "terrorist groups" that it says have taken advantage of protests.
The uprising in Syria was sparked by the arrest of teenagers who scrawled anti-regime graffiti on a wall in Daraa. Protests spread quickly across the nation of some 23 million people.
Assad inherited power from his father in 2000.
Associated Press writers Zeina Karam and Elizabeth A. Kennedy in Beirut and Maggie Michael in Cairo contributed to this report.
Bassem Mroue can be reached at http://twitter.com/bmroue