CORRECTED: Leftist protests push Mexican Congress to side room

Corrects to clarify the last such incident was in 1989

By Miguel Angel Gutierrez and Anahi Rama

MEXICO CITY (Reuters) - Sit-in protests by left-wingers forced Mexico's Congress to relocate on Tuesday for the first time in two decades as political tensions seethed over a government plan to boost private investment in oil.

Conservative and centrist lawmakers crammed into function rooms to press ahead with their agenda after leftists blocking debating chambers since Thursday refused to budge.

"We can't negotiate the unnegotiable, which is the podium," Sen. Santiago Creel, Senate leader for the ruling National Action Party, or PAN, told reporters, as left-wingers in oil worker suits snoozed in chairs in the upper house.

"If they want to stay there, there will be sessions in the other room," he said, as senators shuffled off to a nearby building to discuss the siege, which leftists refuse to break unless the PAN pledges a months-long debate on energy.

Deputies gathered in a conference room to vote on sending President Felipe Calderon on a trip to New Orleans to meet his U.S. and Canadian counterparts as left-wing congressmen also continued to block the lower house podium.

Congress has only been booted out of its main chamber a handful of times in Mexico's history: once when an occupation of Mexico City by French forces in the 19th century forced congressmen to seek alternative premises, and a few times due to fires, the most recent being in 1989.

Leftists, the second force in Congress, are up in arms about Calderon's plan to change the law to allow state oil monopoly Pemex to sweeten oilfield service contracts with bonus fees that could lure foreign partners to deep-sea projects.