President Elect Barack Obama acknowledged yesterday that his campaign pledge to close the prison at Guantanamo Bay will be more of a challenge than he had anticipated.

Many of the estimated 250 people still being held at the military site are suspected terrorists or potential witnesses in cases against them.

"It is more difficult than I think a lot of people realise - and we are going to get it done - but part of the challenge that you have is that you have a bunch of folks that have been detained, many of whom may be very dangerous, who have not been put on trial or have not gone through some adjudication," Obama said in an interview broadcast yesterday, on the seventh anniversary of the opening of the camp in Cuba.

The President Elect said that while some evidence against terrorism suspects may be tainted by the tactics used to obtain it, that did not change the fact they were "people who are intent on blowing us up".

In general, Obama said, the United States had made progress in becoming safer since the September 11 attacks, but dangers persisted. He said national security remained a concern, but added: "We know exactly what they're planning, where they're positioned. If you have a small group of people in today's world with today's technology who are intent on doing harm and are willing to die, that is something that's always going to be a challenge."

Protests took place in London and Madrid to mark the seventh anniversary of the opening of Guantanamo Bay.

About 30 people gathered for a rally outside the US embassy in London.

Former Guantanamo Bay detainee Omar Deghayes was to address the protest. He was released in December 2007.

In Madrid, around 100 demonstrators held a similar protest outside the US embassy in the Spanish capital.

Obama also criticised the Bush administration for lack of oversight in implementing the $700bn federal bailout plan for the reeling US financial system and called for tighter controls on how the remaining funds are spent.

The incoming president's economic team and the Bush administration are consulting about release of the second $350bn allocated under the bailout measure. But the Treasury Department has come under criticism for failing to provide taxpayers and Congress with enough disclosure about the massive injection of money into the financial system.

"Let's lay out very specifically some of the things that we are going to do with the next $350bn," Obama said in the ABC TV interview.

"I think that we can regain the confidence of both Congress and the American people that this is not just money that is being given to banks without any strings attached and nobody knows what happens, but rather that it is targeted very specifically at getting credit flowing again to businesses and families."

He also voiced dissatisfaction with how the first $350bn was spent under the Troubled Asset Relief Programme (Tarp) passed by Congress last autumn.

"I, like many, are disappointed with how the whole Tarp process has unfolded. There hasn't been enough oversight. We found out this week in a report that we are not tracking where this money is going," the President Elect said.

Obama declined to say whether he wants President George W Bush to request the rest of the money before Obama is sworn in on January 20, but he said he has asked his economic team to develop a set of principles to ensure more openness about how the money is spent.

In the interview, Obama also conceded it would be difficult to enforce his pledge to ban politicians from using the money to funding non-essential spending projects for their districts.

But Obama said inaction carries too great a risk. "We can't afford three, four, five, six more months where we're losing half a million jobs per month.