Anti-regime demonstrators have clashed with police outside the Syrian embassy in London, following reports that nearly 300 people have been killed by government forces in the Syrian city of Homs.

Tank shells and mortars rained down on the restive city as pro-government forces look to crush the opposition in the face of mounting international pressure.

According to opposition group, the Syrian Network for Human Rights, the army and security forces launched an attack just before midnight, bombarding the "district with more than 225 mortar shells”.

The group say that 285 have been killed and 950 injured so far. The numbers are unverified due to restrictions on foreign journalists in Syria.

Following the attack, William Hague accused the Syrian regime of "cold-blooded cynicism".

US President Barack Obama has condemned the "unspeakable assault against the people of Homs".

The offensive comes as the United Nations Security Council prepares to vote on a resolution calling for president Bashar al-Assad to step aside.

In New York, Western powers have ruled out military action but want the United Nations to endorse an Arab League plan that calls on the president to hand power over to Syria's vice president.

The move towards a vote came after US secretary of state Hillary Clinton spoke by telephone with Russian foreign minister Sergey Lavrov in a bid to overcome Russian opposition to any statement that explicitly calls for regime change or military intervention in Syria.

According to a State Department spokesman, Mrs Clinton and Mr Lavrov agreed that American and Russian diplomats would continue work on a Syria resolution and were planning to meet for more talks over the weekend.

Russia's deputy foreign minister Gennady Gatilov said Moscow could not support the resolution in its current form, but expressed optimism that an agreement could be reached, state news agency RIA Novosti reported.

Earlier this week, Foreign Secretary William Hague made an impassioned plea for the UN to support moves to bring a peaceful resolution to the unrest.

At a Security Council meeting in New York, Hague said the risk of civil war was intensifying and the threat to the stability of the region growing.

The foreign secretary said: "With each day that passes, finding a way back from the brink will be harder and innocent lives will be needlessly and wrongfully lost, deaths which this council could help to avert by acting in a united manner."

In London on Saturday morning five people were arrested after gaining entry to the Syrian embassy. Around 150 demonstrators descended on the building in Belgrave Square.

By Saturday afternoon clashes had broken out between demonstrators and police as the activists attempted to storm the embassy.

Rocks and bottles were thrown, shattering windows, as police attempted to quell heated scenes.

One protester said his goal was to eject the regime's ambassador from the UK. In a message directed at David Cameron he said: "You need to get rid of the ambassador."

Another protester of Syrian descent said they were attempting to "storm the embassy" because they could no longer accept the regime's presence in London.

Police moved to re-enforce the building, including the erection of metal barriers. Sandbags were dropped into the fortifications in an attempt to hold back the angry demonstration.

The Syrian embassy, London

Tory MP Andrew Rosindell, a member of the foreign affairs committee, told Sky News that the government should consider expelling the Syrian ambassador.

"The murderous actions we are now witnessing in Syria requires the foreign secretary to urgently consider whether ambassador should remain in London," he said.

"I do think he should go, the regime in Syria should understand such actions can not be tolerated."

A Metropolitan Police spokesman said a number of people were arrested for public order offences after the demonstration broke out at 2am and "appropriate policing" was in place. Windows were reportedly smashed at the building as the protest broke out.

The Metropolitan Police spokesman said: "Officers are at the scene and appropriate policing is in place. Five males gained entry to the building. They have now been arrested."

Ronan McNern, a supporter of the Occupy London movement, attended the protest.

He said he went to the scene out of solidarity for the demonstrators and to bring them sweets after being called by another Occupy London campaigner. Describing the scenes, he said: "The protesters are being held on the other side of the road from the embassy.

"There are 150 people surrounded by a ring of about 60 police officers, who are carrying truncheons. There are also about 12 police vans. It seems all right at the moment. The protesters are keeping their spirits up by singing, dancing and playing drums, and they seem to be free to leave the ring. Some of them are waving Syrian flags."

Paint appeared to have been thrown on a door of the embassy, he added.

According to the UN, more than 5,500 people have died in the country since pro-democracy campaigners took to the streets last year protesting against the Assad regime.