Mexico City ? Supporters of leftist Andres Manuel Lopez Obrador pledged Tuesday to place conservative Felipe Calderon “under siege” if he is declared the winner of the disputed presidential elections.

Supporters of Calderon, who holds a slight advantage in official vote counts on the July 2 race, meanwhile accused Lopez Obrador of wanting to make blood flow in the conflict.

The heightened rhetoric came one day after the first violent incident in a month of protests, when police and protesters clashed outside the Congress building in Mexico City. The leftists plan another march on Congress on Sept. 1, and plan to continue blockading some streets in Mexico City through Sept. 16, the date of the traditional Independence Day parade.

“He will be a president under siege … he will not be able to operate outside his office,” Gerardo Fernandez, spokesman for Lopez Obrador’s Democratic Revolution Party, said of Calderon.

Lopez Obrador has said he “will not accept” Calderon as president. He claims the race was marred by fraud and has demanded a full recount.