Craigavon Bridge in Londonderry has been blocked by about 150 people protesting against the planned visit by George Bush to Northern Ireland.
Blair and Bush last met at Camp David last week
They were removed by police as they stopped traffic on the main route across the River Foyle into the city centre.
The US president will fly into the province on Monday for talks with Prime Minister Tony Blair.
The meeting will also cover the peace processes in the Middle East and Northern Ireland.
Irish premier Bertie Ahern is expected to join the talks on Northern Ireland, as are the leadership of the pro-Good Friday Agreement parties.
Sinn Fein leader Gerry Adams said he was "pleased" by the news of the meeting at Hillsborough Castle, County Down, but the SDLP said it was "perturbed".
SDLP leader Mark Durkan said: "Hillsborough is being used for meetings to plan the further prosecution of
the war in Iraq as well as to hold separate discussions on our own situation.
"I cannot disguise my personal unhappiness at this, given my own opposition to this war and my concern for the integrity of our own peace process."
Post-war Iraq administration
Ulster Unionist leader David Trimble urged Mr Bush and Mr Blair not to give "mixed messages which would only give succour to Irish republicans".
Anti-war protesters in Ireland are organising buses to stage a demonstration in Belfast to coincide with the talks.
Spokesman Richard Boyd Barrett said: "How can Bush come to Ireland talking about peace while his army is
subjecting the people of Baghdad to a medieval siege with 21st century weaponry?"
The meeting is the third summit between Mr Bush and Mr Blair in recent weeks.
They met in the Azores ahead of the outbreak of war in Iraq, and Mr Blair travelled to the US for talks at Camp David last week.
Downing Street said the two previous meetings had been "very helpful in the development of our strategy on the military,
diplomatic and humanitarian fronts".
At a time when some are accusing the president and the prime minister of being warmongers, it is obviously helpful for both leaders to stress their support for peace
The men are expected to discuss the administration plans for post-war Iraq, which is a point of disagreement for the US and UK.
The UK has been pushing for the United Nations to be in charge of running the country, while the US has said it wants to retain control.
security adviser, Condoleezza Rice, said on Friday the US and
its war allies had given "life and blood" to the Iraq war, so the coalition "would have the leading role".
White House spokesman Ari Fleischer said the trip's primary focus would be Iraq.
talk about the humanitarian relief efforts. They will talk
about reconstruction. They will talk about the role of the
Another of the summit's aims will be to kickstart the Northern Ireland peace process, ahead of the fifth anniversary of the signing of the Good Friday peace agreement.
Mr Blair and Mr Ahern are due to set out new proposals for a "make or break" implementation agreement later in the week.
A Downing Street spokesman said: "It is an important week in the Northern
Ireland peace process.
"It will be useful to get the US president's support for our efforts to
encourage the leaders to the acts of completion the prime minister has
Northern Ireland was an example "of how peace can be taken forward in seemingly impossible
situations", he said.