Marie Colvin dies along with photographer Remi Ochlik after shell hits house in Homs, while three activists are also killed ITN

The deaths of the veteran Sunday Times correspondent Marie Colvin and the French photographer Rémi Ochlik amid a rising toll of civilian victims in Syria have prompted renewed calls for an end to the regime of Bashar al-Assad.

Their deaths came on a day in which, according to activists, more than 80 people were killed in the besieged district of Baba Amr in Homs, which has been under daily attack by the Syrian army for three weeks.

The French president, Nicolas Sarkozy, called the journalists' deaths an assassination and said the Assad era had to end.

"That's enough now," he said. "This regime must go and there is no reason that Syrians don't have the right to live their lives and choose their destiny freely. If journalists were not there, the massacres would be a lot worse."

The foreign secretary, William Hague, said the deaths were "a terrible reminder of the suffering of the Syrian people – scores of whom are dying every day". He added: "Marie and Rémi died bringing us the truth about what is happening to the people of Homs. Governments around the world have the responsibility to act upon that truth – and to redouble our efforts to stop the Assad regime's despicable campaign of terror in Syria."

The Syrian ambassador to London was later summoned to the Foreign Office ,where officials told Dr Sami Khiyami the UK government was "horrified" by the violence in Homs.

Political director Sir Geoffrey Adams said he expected immediate arrangements to be put in place for Colvin's body to be repatriated, as well as for the medical treatment of British photographer Paul Conroy, who was also injured in the shelling.

A Foreign Office spokesman said: "Sir Geoffrey stressed that the British government was horrified by the continuing unacceptable violence in Homs, which has been under attack for 19 days.

"He noted that today alone the world had witnessed the death of more than 60 civilians, including children, on the single street of al-Hakoura in the Baba Amr neighbourhood.

"Our clear demand was for the violence to stop immediately. The Syrian authorities must implement the undertakings they had given to the Arab League, halt all violence against civilians, and start an orderly political transition before a single further death took place."

David Cameron told the Commons Colvin was a "talented and respected foreign correspondent" and her death was "a desperately sad reminder of the risks journalists take to inform the world of what is happening and the dreadful events in Syria".

Colvin and Ochlik were killed after an artillery shell hit the house in which they were staying. Three other foreign reporters, as well as seven activists from Baba Amr, were wounded on Wednesday. One of the injured, freelance photographer Paul Conroy, was travelling with Colvin.

Edith Bouvier, a freelance journalist working for the French paper Le Figaro, suffered serious leg injuries in the attack. Activists warned that she was at risk of bleeding to death.

Jean-Pierre Perrin, senior foreign correspondent at the French daily Libération, said he had been with Colvin and other journalists at a makeshift press centre in Homs and had left with her several days ago after being warned that the army was preparing an offensive and that journalists could be targeted. Colvin waited, decided the offensive against the press centre had not happened and returned to Homs.

Perrin told Libération the press centre, which had a generator and a patchy internet connection, was the only means of telling the world what was happening. "If the press centre were destroyed, there would be no more information out of Homs."

He said the army recommended "killing any journalist that stepped on Syrian soil". Journalists had been aware of this, and of reports of intercepted communications between Syrian officers that recommended killing all journalists between the Lebanon border and Homs, and making out they had been killed in combat between terrorist groups.

He said of his departure from Homs with Colvin: "We had been advised to leave the town [of Homs] urgently. We were told, 'If they find you, they will kill you'." So I left with the Sunday Times journalist [Marie Colvin], but later she wanted to go back when she saw the offensive hadn't happened."

In the deadliest time for the media since the uprising began, at least three citizen journalists have also been killed in recent days, in an apparent attempt by the regime to prevent news emerging from Homs. The three had played prominent roles in chronicling the army's assault on Homs.

One of them was video blogger Rami al-Sayed, known as Syria Pioneer, who had uploaded to the internet at least 200 videos of killing and destruction in his neighbourhood.

Colvin, a decorated foreign correspondent with more than 30 years' experience in conflict zones, and Ochlik, who won a World Press Photo award last month, died instantly when the shell struck the safe house provided for them by activists just after 9am. Colvin's body, along with Ochlik's, was recovered four hours later.

Colvin's editor, John Witherow, said in a statement: "Marie was an extraordinary figure in the life of the Sunday Times, driven by a passion to cover wars in the belief that what she did mattered. She believed profoundly that reporting could curtail the excesses of brutal regimes and make the international community take notice. Above all, as we saw in her powerful report last weekend, her thoughts were with the victims of violence.

"Throughout her long career she took risks to fulfil this goal, including being badly injured in Sri Lanka. Nothing seemed to deter her. But she was much more than a war reporter. She was a woman with a tremendous joie de vivre, full of humour and mischief and surrounded by a large circle of friends, all of whom feared the consequences of her bravery."

Colvin and Ochlik had been in Baba Amr for the past weekreporting the bloody siege of opposition-held parts of Syria's third city, which has claimed hundreds of lives and led to a humanitarian crisis. The house in which they were based was next to a hospital and had been the main refuge for all reporters who had made it to Bab al-Amr in the face of a relentless barrage by regime forces.

An activist for the campaigning group Avaaz who saw the attack said: "I left the house after it got struck and headed to a house across the street. The shelling continues and the bodies of the journalists are still on the ground. We can't get them out because of the intensity of the shelling even though we're only a few metres away from them."

Another witness said rockets continued to rain down on the area as the wounded tried to escape the bombed two-storey house. A graphic video posted on the internet showed the house in ruins – a scale of damage that could only have been caused by a heavy artillery round. Two bodies were visible in the rubble.

Three of the wounded were in a serious condition and in urgent need of treatment.

They faced a long and perilous drive to the Lebanese border where Red Cross officials were preparing to meet them.

The foreign editor of the Times, Richard Beeston, said on Twitter: "Terrible news about Marie Colvin. First worked with her Beirut 85. Most courageous, glamorous foreign corr I have ever met. Tragic loss."

Colvin used a web forum to make what is believed to be her last post on Tuesday. "I think the reports of my survival may be exaggerated," she wrote. "In Baba Amr. Sickening, cannot understand how the world can stand by and I should be hardened by now. Watched a baby die today. Shrapnel, doctors could do nothing. His little tummy just heaved and heaved until he stopped. Feeling helpless. As well as cold! Will keep trying to get out the information."

Last nightOn Tuesday night Sayed also lodged a final missive. "Baba Amr is being exterminated. Do not tell me our hearts are with you because I know that. We need campaigns everywhere across the world and inside the country. People should protest in front of embassies and everywhere. Because in hours, there will be no more Baba Amr. And I expect this message to be my last."