Riot police prevent supporters of Andres Manuel Lopez Obrador from getting to National Congress in Mexico City

Mexican riot police have fired tear gas to disperse protesters who massed outside Congress to demand a full recount in last month's disputed presidential election.

Officials say the tear gas was fired to end a brief clash Monday between police and supporters of leftist presidential candidate Andres Manuel Lopez Obrador.

The former Mexico City mayor has alleged fraud in the election that he narrowly lost to his rival, conservative Felipe Calderon, and has called for all the votes to be recounted.

The Federal Electoral Tribunal ordered a recount in nine percent of the country's 130,000 polling places. The recount has been completed but the results have not been made public.

On Sunday, Lopez Obrador threatened more protests if electoral officials declare Calderon the winner. The court has until September 6 to declare a president-elect.

The person named president will succeed Vicente Fox, who steps down in December after six years in office. By law, President Fox cannot seek a second term.

Some information for this report was provided by AP and Reuters.