A day after President Bashar al-Assad scrapped decades of emergency rule, his forces used live ammunition against demonstrators in several towns and cities nationwide, witnesses and activists told AFP by telephone.
The official SANA news agency said security forces intervened using tear gas and water cannon to "prevent clashes" between protesters and passers-by.
SANA spoke of 10 dead, including police.
"The Syrian security forces committed massacres in several towns and regions today (Friday), so far killing 72 people and wounding hundreds," said the London-based Syrian Human Rights Committee in a statement received by AFP.
Several rights activists also published provisional lists recording the deaths of more than 70 people.
Earlier, witnesses and activists said at least 38 people were killed on one of the bloodiest days since pro-democracy protests erupted in Syria in mid-March.
Ammar Qurabi, chairman of the Syrian National Human Rights Organisation, spoke to AFP of "49 deaths and 20 people reported missing" on Friday.
Rights activists and groups reported 15 killed at Ezreh in the southern Daraa province, hub of the anti-regime protests. They also reported 15 dead in the central city of Homs.
They said another 30 people were killed near Damascus, and the rest were killed in other towns.
Two deaths were earlier reported in the northern city of Hama, site of a government-sponsored massacre of Islamists in 1982, and two more in the main Syrian port of Latakia.
The official SANA news agency spoke of "eight dead and 20 wounded including members of the security forces in an attack by armed criminals in Ezreh," and "two policemen killed and 11 wounded in Homs and Damascus by armed groups."
Friday's death toll was one of the bloodiest in protests for democratic change -- the first since emergency rule was imposed by the ruling Baath Party when it seized power in 1963.
"The Syrian authorities have again responded to peaceful calls for change with bullets and batons," said Malcolm Smart, Amnesty International's Middle East and North Africa director.
"They must immediately halt their attacks on peaceful protesters and instead allow Syrians to gather freely as international law demands."
A coalition of protesters from across Syria earlier issued a list of demands in a statement blasting "attempts by the Syrian tyrannical machine to thwart and circumvent the acquisition of our basic rights and needs."
The White House condemned Friday's violence, calling for action from all sides. "We call on the Syrian government to cease and desist from the use of violence, we call on all sides to cease and desist from the use of violence," it said.
British Foreign Secretary William Hague condemned the "unacceptable" killing of demonstrators and called for emergency law to be lifted in practice, not just in theory.
"Political reforms should be brought forward and implemented without delay. The emergency law should be lifted in practice, not just in word," he said.
SANA said security forces "intervened" using tear gas and water cannon "to prevent clashes between protesters and citizens and protect public property."
Thousands of protesters swarmed the mainly Kurdish city of Qamishli in northeastern Syria and Banias in the north.
Protesters in Zabadani, northwest of Damascus, called for Assad's regime to quit and also chanted slogans hostile to key regional Syria allies Iran and the Lebanese Shiite group Hezbollah, witnesses said.
About 200 people chanting "freedom, freedom" marched in central Damascus but were quickly dispersed by police, said an activist, while 100 who protested in the central city of Raqqa were scattered by baton-wielding pro-regime supporters, lawyer Abdullah Khalil said.
Assad, in power since replacing his father Hafez as president in 2000, issued decrees Thursday scrapping emergency rule, abolishing the state security court and allowing citizens to hold peaceful demonstrations.
But his detractors said the moves were not enough, and the so-called Syrian Local Coordinating Committees of protesters made a raft of demands, urging a halt to the torture, killings and arrests of protesters.
Friday's protests came after a call by Facebook group The Syrian Revolution 2011 for rallies spanning the Christian and Muslim faiths on "Good Friday," which commemorates Jesus Christ's crucifixion.
Friday is also the Muslim day of rest when the biggest demonstrations have been staged across Syria after weekly prayers.
Assad's scrapping of emergency rule and abolition of the state security court was the latest in a series of concessions over the past month to placate protesters.
Before Friday, security forces and plain-clothes police had killed about 220 people in a brutal crackdown on the protests.