The bombardment of Homs will be judged by history as a massacre comparable to Rwanda or Srebrenica, says a British photographer injured in the attacks.
Paul Conroy, 47, told BBC's Newsnight he had witnessed "a systematic slaughter of the civilian population".
In all his experience of war zones, he had never seen anything on this level, he said, and in 10 years, the world would be "wringing its hands" in shame.
Journalist Marie Colvin, killed in the shelling, was "the best of the best".
"I don't know anyone who had her tenacity and bravery," he said of his Sunday Times colleague.
Mr Conroy, who was smuggled to Lebanon by Syrian rebels, paid tribute to the "heroes" who helped him escape, but he feared for their safety and for all the Syrian people.
"We left behind what I fear is going to be the next Rwanda, the next Srebrenica."
The bombing of Baba Amr - the rebel district of Homs which has received the brunt of the Syrian authorities' attacks - is indiscriminate, he said.
"There are no military targets, it's pure and systematic slaughter of a civilian population. The only reason those shells are going in are to eliminate the people and buildings of Baba Amr."
Conroy spent days stranded in Baba Amr with shrapnel wounds picked up in a rocket attack that killed Colvin and French journalist Remi Ochlik.
Speaking from his hospital bed in London, Conroy described conditions in Homs as catastrophic, with people living in "bombed-out wrecks" and "waiting to die".
The next phase in the conflict will not be televised but it will happen, he said.
Urging the international community to act, he said that when Baba Amr fell, the regime would turn to other areas.
"When Baba Amr is finished, and I think it's almost there now, we've watched it happen, they're going to move on, they're going to move into the countryside, the towns and there will be no witnesses," he said.
"Women, children, old men, young people will just cease to exist, they will cease to exist and in 10 years we will have an investigation and people will say 'how did this happen?'"
Conroy, originally from Liverpool but now living in Devon, said the moment his building was hit was "was traumatic, instant chaos".
"A few shells had hit the house and the final shell that killed Marie and Remi, my friend, everything went black, I felt a huge pressure in my leg... I realised it was bad, stuck a tourniquet on and essentially tried to crawl out the house where I found Marie."
He said they were taken to a field hospital which was "a living room and a kitchen table essentially," where doctors had limited supplies.
The building was taking direct hits in intense shelling which lasted 14 hours a day, he said.
"It was intense to start with but as the week went on we were living minute-by-minute in that room, literally you could hear the shells coming in.
"We also had the medics coming in saying 'We've just lost another 10 men, another five children,' so while we were in this situation we were getting daily, hourly, minute-by-minute reports of the slaughter outside, which was quite un-nerving."
Meanwhile, the International Committee of the Red Cross said it had received the bodies of Colvin and Ochlik, and taken them to Damascus.
The Red Cross said on Saturday it will try again to be allowed access to the devastated Baba Amr district of the Syrian city of Homs, after being blocked on Friday.
It said it was "unacceptable" that its seven-lorry convoy had been stopped.
There have been reports of revenge killings and summary executions by Syrian forces since rebel fighters pulled out of Baba Amr on Thursday.