Riot-prepared police, backed by a helicopter overhead, prevented demonstrators on Monday from approaching the US embassy in Brussels.
But the protest remained peaceful.
"Bush killer" was among slogans chanted by the protestors, organised by the Stop Bush umbrella group which includes some 80 pacifist, human rights and environmental organisations.
Two queues of police clad in helmets and with shields and batons separated the demonstrators from the embassy compound, where the US president is staying during a two-day stop in Brussels.
The organisers said some 4000 to 5000 people took part, but police put the figure closer to 4000.
One man detained
One young man who threw some eggs at police was detained during the rally which otherwise went off without incident.
More demonstrations are planned
for Tuesday afternoon
Head of French Belgian Amnesty International Philippe Hensmans said he was concerned about the erosion of civil rights in the United States since the 11 September 2001 attacks.
"If George Bush wants to talk to the EU, that's good. But the Europeans have to be ready to remind him that we can't break fundamental rights," he said.
"We are demonstrating for peace and against the economic policies of George Bush, which are based on resorting to force," said Claudio Arduini, a young member of the Flemish party flamand Spirit, which is linked to the ruling liberal-socialist government.
More demonstrations are planned on Tuesday afternoon when Bush is due to meet with the heads of state and government from the 25-member European Union.