US President Barack Obama has warned of "hard days ahead" at a Nato summit in Chicago dominated by the issue of withdrawal from Afghanistan. France's new President Francois Hollande has again said he will pull French troops out by the end of 2012, nearly two years ahead of schedule. Nato is preparing to hand over security to Afghan forces by the end of 2014. Afghan President Hamid Karzai said his country was fully aware of its responsibilities. The summit is taking place amid heavy security. Outside the conference, riot police clashed with demonstrators protesting about war, climate change and a wide range of other issues. Witnesses said some had blood streaming down their faces. Police were seen carrying some people away from the scene. The protest march through Chicago had been largely peaceful until the end when a small group of demonstrators tried to break through police lines and reach the lakeside convention centre where the summit is being held.