By Catherine Bremer

MEXICO CITY, Feb 24 (Reuters) - Eighteen months after he crippled the capital with protests over a 2006 election defeat, Mexican leftist Andres Manuel Lopez Obrador was back on the street on Sunday to protect the state monopoly on oil.

Obrador, who says he was robbed of the presidency by electoral fraud in July 2006, mustered thousands of protesters outside state oil firm Pemex's 52-story headquarters to slam fledgling proposals to allow private investment in oil.

The firebrand leftist told the crowd that if rallies did not work, they would take over the Mexico City airport, highways and the stock exchange and hold a national strike.

"We do not accept anything to do with privatization of Pemex or sharing oil profits," he told a cheering crowd.

Lopez Obrador has seized on a planned oil sector reform as a new rallying point for protests against President Felipe Calderon's conservative government.

Calderon wants to pass an energy law by April that would give Pemex more autonomy and possibly allow joint ventures in deepwater oil fields that straddle the U.S. maritime border. Mexico, a top three supplier of crude oil to the United States, saw oil exports slip last year to their lowest level since 2002.

Public support for Lopez Obrador's new cause could determine whether the man whose presidential ambitions rattled Wall Street in 2006 still has a political future and could also set the tone for Mexico's rudderless left for the next few years.

"We're against privatizing Pemex. Lopez Obrador is the only one who can fight against this," said Saul Monsalvo, a graphic designer who arrived at the protest by bus from the neighboring State of Mexico.