MEXICO CITY (Reuters) - Mexican riot police used tear gas and clubs to drive back leftist legislators and supporters protesting outside Congress on Monday in the first violent clash over a fiercely contested presidential election. ADVERTISEMENT Several lawmakers from the left-wing party whose presidential candidate narrowly lost the July 2 election were slightly injured when police swept through their protest camp. Demonstrators threw rocks back at the federal police lines. It was the first time the government has deployed police to break up protests that began days after the election and have until now been peaceful. "They hit us all,...