Europe is more vulnerable to an al-Qaeda attack because of its proximity to the group's operating regions, Barack Obama has said. The US President and his wife Michelle earlier arrived in Strasbourg to cheers from crowds of people gathered to see world leaders arrive in the French city for Nato's 60th anniversary. Mr and Mrs Obama were greeted by well-wishers squashed behind security barriers and he received a kiss from a woman in the crowd. Then the first encounter between two of the world's most glamorous women, Mrs Obama and her glamorous French counterpart, Carla Bruni, took place at the elegant Palais de Rohan. The Nato summit - which Prime Minister Gordon Brown is also attending fresh from his G20 summit success - is co-hosted by Strasbourg and the German cities Baden Baden and Kehl, on the other side of the Rhine river. After talks with French President Nicolas Sarkozy he held a US-style "town hall" meeting of young French and Germans where Mr Obama said: "...It is probably more likely that al-Qaeda would be able to launch a serious terrorist attack on Europe than on the United States because of proximity." He also warned global nations against neglecting their response to climate change or sinking into trade protectionism in response to the financial crisis. Mr Obama told his audience: "Trade wars have no victors. We cannot give up on open markets, even as we work to ensure that trade is both free and fair." And he said "time is running out" in the race to contain damage to the environment blamed on climate change. He will later hop across the border for discussions with German Chancellor Angela Merkel. On the top of the agenda will be the worsening crisis in Afghanistan and Mr Obama will try to secure Nato backing for a new strategy he unveiled last week aimed to try to get a grip on rising violence by Taliban militants driven from power in 2001 but never completely defeated. It broadens the focus to include Pakistan and puts the highest priority on the defeat of al-Qaeda militants who Mr Obama says are plotting new attacks on the US. Earlier, violence flared in Strasbourg as protesters clashed with riot police in running battles ahead of the summit. Officers fired teargas and rubber bullets into the crowd as the violence intensified and 300 people were arrested. The city has been virtually shut down as troops and police mount one of their biggest security operations to protect thousands of delegates from 26 countries.