Thousands of protesters returned to the streets of Damascus as funerals for those killed in a reported massacre by government forces last Friday were held in a suburb of the capital city.

Human rights groups have claimed at least 15 people were shot dead on Friday during pro-reform demonstrations in the neighbourhood of Douma, eight miles north-east of the centre of the Syrian capital. Some non-governmental organisations claim up to 22 protesters may have been killed, with more than 100 wounded, including 20 in a critical condition.

Angry mourners chanted "down with the regime" as eight of Friday's victims, believed to have been targeted by government snipers, were buried.

"This was the systematic killing of peaceful and unarmed citizens by security forces," said Radwan Ziadeh, head of the Damascus Centre for Human Rights, one of several organisations that has collated matching witness accounts of the incident.

Witnesses told the Guardian that thugs were bussed in by government forces to attack demonstrators. Journalists and diplomats were prevented from reaching the area over the weekend, and phone lines to Damascus have been disrupted.

Details of the bloodshed in Douma emerged as beleaguered Syrian president Bashar al-Assad appointed former agriculture minister Adel Safar to form a new government. Assad fired his cabinet last Tuesday in an unsuccessful attempt to quell unrest.

On Friday, thousands of Syrians marched across the country in protest at the failure of the president to make real reforms. Shots were reportedly fired in the cities of Daraa and Homs as well as in Douma.

Human Rights Watch condemned the shooting of peaceful protesters throughout the unrest which has swept Syria in the aftermath of revolutions in Tunisia and Egypt. It said Assad's promises of investigations, reiterated in a speech on Thursday, "rang hollow when security forces are still shooting at protesters".

UN secretary general Ban Ki-moon said he was "deeply concerned" by reports of civilian casualties as the known death toll in Syria climbed above 100.

An official government source told Sana, the state news agency, that security forces were not responsible for the violence in Douma and blamed the deaths on an "armed group" that opened fire from rooftops on protesters and policemen.

State television said "some of the demonstrators had daubed their clothes with red dye to make foreign reporters believe that they had been injured".

But witnesses told human rights groups that snipers opened fire on civilians, including those trying to retrieve dead bodies. Families of the victims claim the authorities are withholding corpses to prevent more funerals taking place. Muslims, who are the religious majority in Syria, believe the dead should be buried as soon as possible.

Crowds at Friday's protests were swelled by residents of Al Tel, a separate town which held its own large anti-government rallies the previous week. Protest organisers said they had travelled to Douma after a member of the ruling Ba'ath party warned them

snipers would be deployed to combat any further protests in Al Tel. The claim could not be independently verified.

Friday's crackdown follows a similarly premeditated attack in the southern city of Deraa, where protests first broke out. On 23 March, government forces cut the internet and electricity before opening fire on unarmed protesters at the Omari mosque, the focal point of protests in the city.

On Saturday, authorities carried out a series of arrests as part of an ongoing crackdown against the uprising. In a joint statement, eight human rights groups said 46 people were arrested in raids in the towns of Deraa, Douma and Homs, although the government simultaneously released some of those previously detained.

Yusef Abu Rumiyeh, a member of parliament for Deraa, denounced security forces for opening fire on his constituents "without pity" and criticised Assad for not offering his condolences.

The appointment of Safar as prime minister is unlikely to appease demonstrators, or please those who previously supported Assad. "We saw him as a reformer," said one man in Damascus, who described himself as neutral before the latest events. "But the speech and the killings have made me keen to join the protests."

Assad has fuelled further discontent by blaming recent unrest on foreign conspirators and failing to lift the country's draconian emergency law, in place since 1963.

Protests have been planned to coincide with Thursday's anniversary of Assad's ruling Ba'ath party. "We will make it the day of its death ... and we will walk to all the party's headquarters and protest in front of them," said the Syria Revolution 2011 protest group on its Facebook page. "We will paralyse the state's joints until [Al-Assad] appears and says 'I understand you.'"

Katherine Marsh is the pseudonym of a journalist working in Damascus.