French police held back protesters with tear gas and detained two dozen people in a pre-dawn clash before a summit of leaders of 28 NATO nations Saturday on the French-German border. Some 1,800 protesters left their camp in the south of the French city of Strasbourg at about 4 a.m. and headed north through deserted streets to the summit site, surveyed from above by several police helicopters shining searchlights. The site is ringed with heavy security, and riot police kept them back with tear gas and barricades. The police headquarters for the Bas-Rhin region said 25 people were detained in the incident. US President Barack Obama and the 27 other leaders are meeting to discuss the war in Afghanistan and NATO's future in a summit jointly hosted by Strasbourg and the German towns of Kehl and Baden-Baden.