Amnesty International called for an independent inquiry into the circumstances of the death of Muammar Gaddafi, who appears to have been captured alive before being shot.
Footage broadcast by Arab satellite television stations showed the ousted Libyan dictator being manhandled by pro-government fighters.
He is bloodied and looks weak in the images but can be heard shouting at his captors as he is bundled towards a car. Soon afterwards, he was shot.
Human rights campaign group Amnesty called for the National Transitional Council (NTC) to publish the full facts of Gaddafi's death and hold a "full, independent and impartial inquiry to establish the circumstances of Colonel al-Gaddafi's death".
Foreign Secretary William Hague acknowledged that the footage appeared to show that Gaddafi was captured alive and then assassinated.
"Until we're sure I don't want to add to speculation about that. I agree that the footage does suggest that," he told Channel 4 News.
"We would have preferred him to be able to face justice at the International Criminal Court or in a Libyan court for his crimes. We don't approve of extra-judicial killings.
"At the same time we are not going to mourn him - this and the fall of Sirte and Bani Walid is a major opportunity for Libya to be able to move on to what they've fought for all this year, into a free and democratic future."