Mr Brown said the UK government had 'no role' in the decision
Gordon Brown has said he was "repulsed" by the welcome given to the Lockerbie bomber on his return to Libya.
In his first comments since the freeing of Abdelbaset Ali al-Megrahi, the PM said he was "angry" at the jubilant scenes that greeted Megrahi in Tripoli.
Mr Brown said he had no role in the Scottish Government's decision to liberate Megrahi, and declined to say whether he supported the move.
Opposition parties accused the PM of "evasion" and a failure of leadership.
Megrahi, who has terminal cancer, was freed on compassionate grounds last week.
Shortly after the decision, Mr Brown wrote to Libyan leader Colonel Muammar Gaddafi urging that Megrahi be given a low-key reception upon his return to Libya.
But Megrahi, found guilty of killing 270 people in the 1988 Lockerbie bombing, received a rapturous welcome when he touched down in Tripoli.
Mr Brown said: "I was both angry and I was repulsed by the reception that a convicted bomber guilty of a huge terrorist crime received on his return to Libya."
He added that he had made it "absolutely clear" to the Libyan leader when they met in July at the G8 summit in Italy that the decision on freeing Megrahi was a matter for the Scottish Government alone.
I was both angry and I was repulsed by the reception that a convicted bomber guilty of a huge terrorist crime received on his return to Libya
He said the UK could not interfere in what was a "quasi-judicial process" and could have "no control over the final outcome".
Mr Brown stressed that his "first thoughts" were with the families of victims of the Lockerbie atrocity but said he was still committed to working with countries such as Libya to fight terrorism.
"I want to make it absolutely clear, however, that whatever decision was made on compassionate grounds by the Scottish Parliament, our resolve to fight terrorism is absolute."
He added: "Our determination to work with other countries to fight and to root out terrorism is total, and we want to work with countries - even countries like Libya, who have renounced nuclear weapons now and want to join the international community. We want to work with them in the fight against terrorism around the world."
BBC political correspondent Laura Kuenssberg said Mr Brown had chosen his words carefully because of the many sensitivities involved in the case and that No 10 clearly hoped his remarks would draw a line under the matter.
The Scottish National Party administration's decision to approve Megrahi's release has been condemned by the relatives of some Lockerbie victims and by politicians in Edinburgh, Westminster and Washington.
For the Conservatives, shadow foreign secretary William Hague said it was "astonishing" Mr Brown had taken so long to comment on the matter and that the PM was not prepared to address the question of whether Megrahi's release was justified.
"I think it's part of a continuing failure of leadership that he is unable to say that he will defend the decision, or that he wasn't happy with it," he said.
"The prime minister ought to be more forthright about this decision."
Liberal Democrat foreign affairs spokesman Ed Davey said: "Gordon Brown's comments on Lockerbie are a masterclass in evasion.
"When a decision is made by another politician, and has such grave international consequences, the prime minister's refusal to say whether or not he supports it almost amounts to negligence.
"It is hard to see why he can't tell us what he thinks of the decision to release a man who has been convicted of the worst terrorist attack in British history.
"As long as Gordon Brown remains silent on this issue, people will suspect he has something to hide."
Ministers have denied the decision to free Megrahi, who served 10 years of his life sentence for the bombing of a Pan Am jet over Lockerbie, was linked to trade deals with Tripoli.
The Conservatives have called for notes of recent meetings between government ministers and their Libyan counterparts to be released to prove that the decision was not influenced by "commercial considerations".
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