Security forces have reportedly opened fire on demonstrators in and around Syria's southern city of Daraa, killing as many as 20 people.
Witnesses said they died in the village of Sanamein as they tried to march to Daraa, but the reports have not been verified.
Amnesty International has said 55 people have been killed in Daraa in the past week.
Unrest has simmered in the area around the city for more than a week. Now it seems to be spreading and the UK's Foreign Office has said it is "deeply concerned" by the violence.
The UN's secretary-general Ban Ki-Moon called for "maximum restraint" by Syria in a phone call with President Bashar al-Assad.
There are reports of protests in the towns of Homs, Tel, Lattakia and the capital, Damascus.
Dramatic footage apparently taken near Daraa shows crowds scattering to the sound of gunfire before they return to the streets to carry away several people clearly severely injured or dead.
Unconfirmed reports claim several people have been killed.
Other video being posted on YouTube and Facebook shows substantial protests in a number of towns. It is impossible to verify when, how or where the videos were taken.
Syrian authorities are not issuing visas to foreign journalists to allow them to enter the country and report on what is happening.
The escalation follows efforts by President al Assad to defuse the unrest yesterday with promises to consider lifting the decades-old state of emergency.
Assad recently said he believed Syria was immune from unrest in a press interview, but since those comments parts of his country have begun seething with protests.
Added to the long list of demands for political reform there are now furious calls for justice for those killed by his security forces, regarded as martyrs by protesters.
Syria has barely tolerated any dissent for almost five decades of Assad rule, first under Hafez al Assad and then under his son.
Despite promises of reform, the regime has been one of the most authoritarian in the Arab world.
It may now be following the path of Egypt, Tunisia and Libya and on the verge of a general uprising. But it is unclear how widespread the protests are.
The unrest is remarkable because it is so unprecedented but so far it has been restricted to a small number of towns. However, that seems to be changing.