MEXICO CITY (Reuters)  April 7, 2005 - About 150,000 Mexicans poured into the streets on Thursday to support Mexico City's leftist mayor, who furiously decried a drive to knock him out of the 2006 presidential race as an assault on democracy.

Chanting protesters packed the capital's Zocalo, one of the world's biggest squares, in a sea of yellow balloons and homemade banners. Police helicopters hovered above but the mayor repeatedly ordered the crowd to be calm.

Congress will vote on Thursday and is expected to lift the legal immunity of Andres Manuel Lopez Obrador so he can face a contempt of court charge. If impeached, he would likely be fired as mayor, go to jail pending trial, and he could be banned from next year's election.

"We are about to see an attack against the democratic advances that have been won with so much sacrifice by the people of Mexico,'' Lopez Obrador said.

Mexico's most popular politician accused President Vicente Fox and business leaders of exploiting the minor legal charges to keep him from leading a "great movement of transformation ... for a new economy and new politics.''

Described by critics as a populist like Venezuelan President Hugo Chavez, Lopez Obrador promises to reduce the huge income gap in Mexico if he becomes president. Buoyed by the popularity of public works and welfare programs in Mexico City, he holds a double-digit lead in opinion polls for the 2006 vote.

The Mexico City crowd reflected Lopez Obrador's broad appeal, with indigenous women in bright red tunics and old men in business suits standing side by side. A man in a cage wearing a plastic mask resembling Fox was paraded through the square to shouts of "Burn him! Burn him!''

Just as the march was starting, Fox left on a plane to attend the pope's funeral in Italy.

Local media and observers estimated the crowd numbered about 150,000 people.

JITTERY FINANCIAL MARKETS

The case has split Mexico and shaken investors concerned about turmoil or violence in a major oil-producing nation where the United States and European countries like Spain have large business and strategic interests.

Mexico only established full democracy during elections in 2000 when Fox ended 71 years of one-party rule, and questions have been raised about how the country will handle a political crisis.

Fears of turmoil have battered Mexican stocks, which suffered their biggest loss of the year on Tuesday but surged 1.5 percent on Thursday morning as the march was peaceful.

Lopez Obrador is expected to lose the vote in Congress's lower house, where Fox's National Action Party and more than 100 deputies from the main opposition Institutional Revolutionary Party are set to vote against him.

The vote is expected around the early evening, and Lopez Obrador plans to defend himself before the chamber.

The mayor repeatedly pleaded with the crowd to avoid violence and not to proceed to Congress, which was blocked off by giant metal barriers and guarded by hordes of police on horseback.

"With this great provocation that is the impeachment, they are betting that we will act irresponsibly, that we'll lose our head,'' Lopez Obrador said. "We cannot fall into that trap.''

Lopez Obrador is accused of disobeyeing a judge's order in 2001 to halt work on a road to a hospital through a disputed plot of expropriated land on the outskirts of the city.

"He could be a great president,'' said Esmeralda Figueroa, who came in from the edge of the city for the protest. "He didn't do anything wrong, and if they kick him out it will be a great heist.''

Pollsters say his popularity would soar if he is jailed because he would look like a martyr for democracy.

"They say that if they throw him in prison he is going to be the national hero of the 21st century. I totally agree with that,'' said Vicente Licona of the Indemerc Harris Interactive poll group.