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.

Lopez Obrador's resurgence into the limelight after a year licking his wounds comes three weeks before his Party of the Democratic Revolution, or PRD, will elect a new leader, with an eye on the 2012 presidential election.

Sen. Jesus Ortega is seen the favorite to win but Lopez Obrador's oil protests could boost the standing of his candidate, Alejandro Encinas, a former Mexico City mayor.

SYMBOL OF SOVEREIGNTY

Pemex says alliances with experienced foreign oil firms could speed its entry into deepwater oil.

But the PRD objects to tampering with the constitution, which gives Pemex sole drilling rights in Mexico. It says allowing foreign alliances smacks of privatization.

"Alliances with foreign firms, under the pretext that we lack technology, are privatization," Lopez Obrador told the crowd.

Mexicans view the oil industry as a symbol of sovereignty since it was expropriated in 1938. A newspaper poll this week showed half the country opposes foreign investment in oil.

"They want to sell it to foreign companies but we're not going to let them," said Hester Palma, 48, selling hotdogs at a stall. "We're going to defend it with our blood."

Still, faced with declining output and reserves, Mexico's other main opposition, the Institutional Revolutionary Party, or PRI, has softened its opposition to energy reform.

PRI Sen. Francisco Labastida, the head of the Senate energy committee, said this week Lopez Obrador was opposing something that had not even been suggested. "It's a movement against ... the ghost of privatization because I haven't seen anyone who wants to sell Pemex," he told La Jornada daily.

Even after his defeat was confirmed by an election tribunal, Lopez Obrador supporters blocked Mexico City streets for weeks with sit-in camps. Today Lopez Obrador calls himself Mexico's "legitimate president" and refuses to recognize Calderon.