Oct. 28 (Bloomberg) -- Mexican opposition lawmakers took over the podium at the lower house of congress, prompting leaders to begin the session at a makeshift stand away from the protest.

Lawmakers allied with former presidential candidate Andres Manuel Lopez Obrador, waving Mexican flags, tried to shut down congress over plans to vote on President Felipe Calderon's initiative to loosen the state oil monopoly. Cesar Duarte Jaquez, head of the chamber, began today's session anyway as Lopez Obrador's supporters set off air horns and rang sirens.

Opposition legislators have taken over the chamber twice this year to protest the energy proposal. Lopez Obrador asked legislators today to add a clause that would prohibit the state- owned oil company from giving outside companies exclusive rights to carry out exploration and production services in a given area.

``I ask you to send the bill back to committees to include an explicit ban on handing over blocks in exclusivity to foreign companies,'' Lopez Obrador, a member of the Party of the Democratic Revolution, told leaders of the lower house before the session.

Javier Gonzalez Garza, head of the Party of the Democratic Revolution in the lower house, also demanded during the session that lawmakers add a clause to the bills prohibiting commercial banks and brokerages from trading ``citizen bonds'' included in the initiative. Calderon proposed the creation of such bonds in April, saying yields would be tied to profit or production at the state oil company, Petroleos Mexicanos.

The lower house plans to vote today on the bills, which would allow performance-based contracts for oil exploration and production for outside companies.

Lopez Obrador's supporters blocked the entrance to the Senate building last week, forcing senators to approve the measures in an alternate site.

To contact the reporters on this story: Adriana Lopez Caraveo in Mexico City at adrianalopez@bloomberg.net; Jens Erik Gould in Mexico City at jgould9@bloomberg.net

Last Updated: October 28, 2008 14:50 EDT