Syrian opposition fighters and civilians trapped in Homs are said to be "preparing for a massacre".
Activists reported people "recording their last wills" as the regime launched a full-scale assault on the city.
"We have nothing to do but pray for them," an activist in Homs, Abu Yaser, was quoted as saying.
It has long been feared that President Bashar al-Assad's forces would launch a ground assault on Homs after almost three weeks of intense shelling failed to quell the opposition.
Power has been cut in the Baba Amr region of the city for almost 24 hours, said the global activist network Avaaz.
The organisation told the Huffington Post UK that video uploads had been cut off by the lack of power, and that anyone known to be uploading material was being targeted by bombardment.
Heavy shelling of the town was reported from three sides of the city, and the Khalidiyeh, Der Ba’alba and al-Bayada neighborhoods were also reported to be under mortar fire.
It was not possible for the Huffington Post UK to confirm the reports independently.
Alice Jay, campaigns director for Avaaz said:
“For four weeks the people of Baba Amr have endured relentless shelling. Now the tanks are rolling into the neighborhood and they are preparing for a massacre.
"The international community has a choice today - bear witness to the total decimation of thousands of lives or intervene now to save the people of Baba Amr."
Earlier it was reported the British photographer wounded the attack which killed Sunday Times reporter Marie Colvin felt the "burden" of knowing people died to get him to safety, it was reported today.
Paul Conroy, 47, was injured last week in the attack which killed Colvin and French photojournalist Remi Ochlik along with many other activists.
The freelance photographer and film-maker from Devon, who suffered three large leg wounds in the attack on 22 February, was smuggled from the country in an operation led by the activist network Avaaz and 35 defectors from the Syrian army in which at least 13 people died.
He was recovering in Lebanon last night, where he spoke to his wife Kate and two sons said the Western Morning News, and was said to be in "good spirits".
But Conroy's wife added: "It is great to be communicating, but he is obviously very concerned for all the people who lost their lives in helping them out.
"It's a real burden on him to know that so many people died."
Meanwhile a new United Nations resolution was said to be in the planning stages after China signalled it wanted to help to reach civilians hit by the violence.
"The international community should create favourable conditions in this regard and provide humanitarian aid to Syria," the Xinhua quoted Chinese Foreign Minister Yang Jiechi as saying.
The new resolution is expected to focus on gaining access for humanitarian organisations to the worst-affected cities, while still recognising the government is the cause of the violence.
The hope is that it will prove harder to reject for Russia and China, who have previously vetoed condemnations of Syria's ruling regime in the Security Council.
Last week US Secretary of State labelled their use of the veto "despicable".
The Reuters news agency was told by a diplomat: "There is a text, though it's not a formal draft resolution yet. It's been drafted by the Americans. It hasn't gone to the full council, just to a small circle of like-minded countries."
At a meeting of the Security Council yesterday the organisation said at least 7,500 people had died in Syria since the start of the uprising in March 2011.