At least 72 protesters have been killed by security forces in Syria, rights groups say - the highest reported death toll in five weeks of unrest there.
Demonstrators were shot, witnesses say, as thousands rallied across the country, a day after a decades-long state of emergency was lifted.
Many deaths reportedly occurred in a village near Deraa in the south, and in a suburb of the capital, Damascus.
US President Barack Obama called for a halt to the "outrageous" violence.
"This outrageous use of violence to quell protests must come to an end now," the president said in a statement.
UK Foreign Secretary William Hague said he was "extremely concerned" by reports of deaths and casualties across Syria and urged restraint on the country's authorities.
"Political reforms should be brought forward and implemented without delay," he said. "The Emergency Law should be lifted in practice, not just in word."
Protesters - said to number tens of thousands - chanted for the overthrow of the regime, Reuters news agency reports.
Video images coming out of Syria show footage of many confrontations where live ammunition was used.
President Bashar al-Assad's lifting of the emergency had been seen as a concession to the protesters.
In their first joint statement since the protests broke out, activists co-ordinating the mass demonstrations demanded the establishment of a democratic political system.
Political unrest in Syria developed after revolts elsewhere in the Arab world, which saw the downfall of the Tunisian and Egyptian presidents and an ongoing civil war in Libya.
At least 260 people are said to have died since it began last month.
'Rain of bullets'
The state news agency Sana said only that security forces had used tear gas and water cannon "to prevent clashes between protesters and citizens and protect public property", and "some" people had been injured.
Deaths were reported by opposition activists and witnesses in Ezra, a village near the flash-point southern town of Deraa, and the Douma suburb of Damascus, as well as the Damascus district of Barzeh, the city of Homs and other areas of the country.
In Ezra, shooting began when protesters marched to the village mayor's office, and one of the dead is said to be a boy of 11.
"Bullets started flying over our heads like heavy rain," a witness was quoted as saying by the Associated Press news agency.
A witness in Douma told Reuters he had helped carry three people with bullet wounds to their legs.
One resident in Homs, a city of 700,000 people in the west, told the BBC she had heard shooting and believed three separate protests were under way in the city.
"The security forces are just dispersing the protesters using live bullets," said the resident, who did not wish to be named.
In Hama, a city in central Syria similar in size to Homs, security forces are said to have also opened fire on a crowd of protesters.
International news organisations are largely refused entry to Syria at the moment, limiting the scope of the information they can gather about events there.
The demands issued by the "Syrian local organising committees" include:
- An end to torture, killings, arrests and violence against demonstrators
- Three days of state-sanctioned mourning for deaths so far
- An independent investigation into the deaths of protesters and judicial proceedings in the light of evidence revealed
- Release of all political prisoners
- Reform of Syria's constitution, including a two-term presidential limit
Before the latest violence, the government insisted it was heeding protesters' demands and President Assad was pushing through a programme of reforms.
Thursday's concessions included abolishing state security courts and allowing peaceful protests but other laws give the government wide-ranging powers to detain activists and suppress dissent.
The new law requires Syrians to seek permission from the interior ministry for demonstrations. Some lawyers have said this continues to restrict the freedom of assembly in the same way as the emergency law.
President Assad said last week there would be no more "excuse" for demonstrations once the state of emergency had been lifted.
Damascus has also accused Islamist militants, or Salafists, of waging an "armed insurrection" in Homs and Baniyas.
Overall, the unrest poses the gravest threat to President Assad's rule since he succeeded his father Hafez 11 years ago.