He said their "remarkable and brilliantly led campaign" - putting on the Live8 concerts and championing the Make Poverty History initiative - gave political leaders at Gleneagles "the support they needed to turn a campaign into a victory".
The Prime Minister rejected claims by sceptics that the additional $50bn (£28bn) in aid to Africa had been recycled from previously announced aid.
"The EU and Japanese commitments are additional, and Canada and the US are agreeing to double their aid. It seems to me that it is new money," said Mr Blair after making a statement to the Commons on the G8 meeting. "This summit of itself cannot end poverty in Africa but it should mark a turning point," he said.
Mr Blair acknowledged that the G8 had failed to deliver all his objectives, but he defended the communiqué on climate change, which was widely condemned by environmental campaigners as too weak.
Charles Kennedy, the Liberal Democrat leader, said the G8 had been " treading water" on climate change. "We will not make progress until we have a target-based, country-based successor to the Kyoto agreement, " said Mr Kennedy.
But Mr Blair rejected his criticism, saying: "You are wrong to say we have been treading water." He said he had secured an agreement on the scientific evidence that climate change was happening, and that human activity was contributing to it. The G8, he added, are committed to continuing a dialogue on a successor to the Kyoto agreement, starting with a meeting in London in November.
However, Mr Blair gave the strongest hint so far that he will be focusing now on securing a breakthrough, at an international meeting on trade in December, on reducing subsidies for exports from developed countries. This would help Africa by stopping goods being dumped on world markets. Michael Howard, the Tory leader, in praising Mr Blair's performance at the G8, attacked trade protectionism by the US and EU as "immoral and hypocritical".
The G8 failed to reach an agreement on an end date for the subsidies but Mr Blair underlined his expectation that he will be able to secure an agreement to cut the subsidies in 2010. His determination to reach a trade deal is contentious because it will require the French to accept reform of the Common Agricultural Policy. However, Cabinet colleagues said last night that progress had been made by securing the agreement of President George Bush to contemplate cuts in US subsidies, if the EU also cuts its subsidies on exports.
On the Middle East, which was also raised at the G8 meeting, Mr Blair said President Bush had told him in private talks that he would give a renewed "push" to reaching a deal. "I am sure President Bush is determined to do this. We discussed it at length at Gleneagles," said Mr Blair.
Mr Blair was pressed by Mr Howard over the abuses by Robert Mugabe's government in Zimbabwe. Mr Blair said the G8 "unanimously deplored recent developments in Zimbabwe". He said what was happening there was "abhorrent" but the issue would only be tackled with pressure from other African countries.
Michael Meacher, the former environment minister, pointed out that debts for Africa were being cancelled on condition that African countries opened up their markets. He also said that the Republican Governor of California, Arnold Schwarzenegger, supported tough action on climate change, but President Bush had resisted it.
Mr Blair said: "If there is a willingness by the [Bush] administration to engage on this, we ought to engage with them and see where we get to."Reuse content