Mexico's Roman Catholic Church closed the country's most important cathedral on Monday after leftist activists burst through its doors in a rowdy weekend protest.

The Church said it will keep the 17th century cathedral, which is a big attraction for foreign tourists in Mexico City's central plaza, shut until the government improves security.

Television images showed an angry mob marching into the cathedral on Sunday and tearing down railings after the cathedral's bells interrupted a political rally outside.

Churchgoers flipped over pews and formed a barricade to keep the activists from approaching the gold altar.

Tensions have risen between Church leaders and the leftist Party of the Democratic Revolution, or PRD, since it narrowly lost last year's presidential election to the conservative National Action Party, which is seen as close to the Catholic Church.

The Church also has sparred with the PRD over recent laws in left-leaning Mexico City to legalize abortion and gay civil unions. Mexico's top churchman, Cardinal Norberto Rivera, last month said he received death threats from members of the party.

"We want the authorities to understand how grave things have been for more than a year," Rivera's spokesman, Hugo Valdemar, told Reuters on Monday.

He said the church had rung its bells on Sunday as it does every day to call for mass.

The PRD denied any involvement in the incident, and party leaders called on members not to break up any more Catholic masses. Mexico is the world's second largest Catholic nation after Brazil.