Media playback is unsupported on your device Media caption Lina Sinjab on the reports of shootings at the protests in the Syrian city of Deraa. Amateur video posted on the internet appear to show demonstrations in Syria on Friday.

At least 23 protesters have been killed during anti-government rallies in the southern Syrian city of Deraa, witnesses have told the BBC.

There are also unconfirmed reports of deaths in Homs, Duma and Harasta, as protests swept the country.

However, state-run Syrian TV said that 19 members of the security forces had been killed "by armed groups" in Deraa.

Deraa has been a focus of unrest since anti-government protests erupted across Syria in mid-March.

The protests have posed an unprecedented challenge to President Bashar al-Assad's 11-year rule. He has offered to consider reforms, but activists say his proposals do not go far enough.

US President Barack Obama said the violence against protesters was "abhorrent".

"I also condemn any use of violence by protesters," he added.

More protests and sit-ins are reported across the country with anger over the deaths in Deraa fuelling protesters. There are concerns that the situation could get worse across the country. Protesters' demands for more political freedom have not been met, and promises made by the leadership are regularly followed by security crackdowns and more killing.

'Pools of blood'

An activist in Deraa told AFP news agency by telephone that demonstrators leaving from three mosques had marched to the city's main court but were confronted by "security forces dressed in civilian clothing" who fired tear gas to disperse them.

Another protester told the BBC's Newshour programme: "After the prayer everybody went... to the town. The police and the army didn't allow them to go through and started shooting at them. We got around 13 people who are dead in the beginning of it and then everybody got attacked. Everybody screaming asking for freedom, at the beginning, but everybody now asking for change in the whole thing."

A resident quoted by Reuters reported seeing "pools of blood and three bodies" in the Mahatta area of Deraa.

Also in Deraa, protesters smashed a stone statue of Basil al-Assad - President Assad's late brother - Reuters reported, citing a witness.

Last month, protesters in Deraa tore down a statue of President Assad's father - former President Hafez al-Assad.

An office of the ruling Baath Party in the Mahatta area was attacked on set on fire, the witness added.

State television said "saboteurs and conspirators" had opened fire in Deraa, killing 19 members of the security forces and wounding 75.

Fresh protests have also been reported in cities including Qamishli, Hasakhe, Idlib, Banyas, Hama and Homs.

The town of Duma, near Damascus, has been sealed off, our correspondent adds, with internet and mobile phone communications blocked.

In Harasta, also on the outskirts of the capital, there are reports of gunfire and of tear gas being fired at protesters.

In northern cities with Kurdish majorities, protesters reportedly chanted "we are calling for freedom and not only for citizenship" in response to President Assad granting citizenship for Kurds in Syria's eastern Hasaka region on Thursday.

In the central city of Homs, thousands are reported to have begun a sit-in. A witness in the coastal city of Latakia said hundreds had taken part in a largely peaceful protest calling for political freedom.

Image caption President Assad has offered to consider reforms, but activists say his proposals do not go far enough

In Hama, security forces used water cannon and smoke bombs to break up a 2,000-strong protest in the old quarter of the city, Reuters reported, citing residents.

The BBC's Lina Sinjab in Damascus says protesters in many towns and cities across Syria took to the streets on Friday calling for greater freedom, but after hearing of the deaths in Deraa the calls were for the regime to be toppled.

She says there is frustration that President Assad has so far not implemented reforms. The fear now is that there will be further bloodshed and crackdowns on protesters, she adds.

Activists estimate that more than 130 people have died in four weeks of clashes, mostly in Deraa and Latakia.

Officials say the death toll is nearer to 30 and they blame the violence on armed groups and foreigners seeking to divide the country.

Syria is one of several countries in the Middle East and North Africa to have been hit by a wave of pro-democracy protests following popular uprisings in Tunisia and Egypt that unseated long-standing autocratic rulers.