US President George W Bush is holding talks with EU leaders at a summit which may be overshadowed by calls for the closure of Guantanamo Bay prison camp.
Mr Bush is welcomed by Austrian counterpart Heinz Fischer
Austrian Chancellor Wolfgang Schuessel, who is hosting the meeting in Vienna, has made it clear he will press Mr Bush to shut down the Cuba detention centre.
For his part, the US president is expected to urge his European allies to push on with sanctions against Iran.
Hundreds of people have marched through Vienna in protest at Mr Bush's visit.
Police have prepared for demonstrations and defused a number of hoax bombs the day before the summit. Tight security includes the closure of roads, airspace and an underground railway station.
The White House said the meeting would be a chance to reaffirm strong ties.
Police dealt with at least three hoax bombs
Correspondents say that, aside from Guantanamo, the meeting is likely to include a number of other contentious issues.
Continuing trade differences between Europe and the US are unlikely to be resolved, says the BBC's Jonathan Beale in Washington.
And EU leaders will be pushing Mr Bush to extend visa-free entry to the US for the 11 member states not covered by the waiver scheme.
But BBC Europe correspondent Tim Franks says that despite the sticking points, European officials say there will be commitments to work together on a number of issues.
The agenda includes:
- proposals to strengthen EU-US co-operation in dealings with energy suppliers, such as Russia
- a plan to create joint teams to find and seize counterfeit goods
- a US plea for Europe to honour pledges of aid to the Iraqi government
- steps to strengthen co-operation on counter-terrorism
- the creation of a joint panel on climate change and clean energy
Austria, which holds the EU's rotating presidency, said last week that the suicides of three detainees at Guantanamo underlined the need to close the prison camp.
EU foreign policy chief, Javier Solana, told the BBC he would endorse Austria's position in talks with Mr Bush, saying Guantanamo Bay was an anomaly.
On the subject of Iran's nuclear programme, before leaving Washington Mr Bush said Europe and the US were united in one of the most difficult challenges facing the world.
Demonstrations began before Mr Bush's arrival
The US president believes a common position on Iran has shown that the US and Europe have put past divisions behind them.
The US has backed European-led efforts to persuade Iran to halt its uranium enrichment programme.
But Mr Bush is now looking to step up the pressure on Tehran if it rejects an international package of incentives.
North Korea's reported plans to test a long-range ballistic missile are also likely to be discussed.
Up to 1,000 people marched through Vienna as the summit began, carrying banners reading "World's No 1 Terrorist" and chanting "We will, we will fight Bush".
A 10,000-strong protest is expected in the capital in the afternoon.
Mr Bush will travel to Hungary after the day's meetings, where he will take part in events to commemorate the 50th anniversary of the country's failed anti-communist uprising.
Mr Bush is the first US president to visit Vienna since Jimmy Carter signed a nuclear disarmament pact with Soviet leader Leonid Brezhnev there in 1979.