Thousands of Catholics marched through Mexico City to the nation's biggest shrine and prayed that the July 2 presidential election does not turn violent.

"We're gathered to pray for peace and unity," Mexico City Cardinal Norberto Rivera Carrera said after leading the June 24 procession, which ended at the Basilica of Our Lady of Guadalupe.

With the two main presidential candidates -- Andres Lopez Obrador of the Democratic Revolution Party and Felipe Calderon of the National Action Party -- in a dead heat, political analysts warned that the loser might organize massive street protests.

In recent weeks, a strike by a teachers' union traditionally allied with the former ruling party has degenerated into violence in the southern state of Oaxaca, and a string of drug gang murders has put states on Mexico's Pacific coast on edge. The weekend of June 24-25, 11 people were murdered in the resort city of Acapulco. Among the dead were four police officers, one of whom was beheaded.

Bishops have been warning that violence could mar the elections and destabilize the country, a worry also shared by financial markets, though the two main presidential candidates promise they are encouraging their followers to be peaceful.

"Hopefully there won't be problems on election day. That's what we're asking Our Lady of Guadalupe," said Ana Laura Aguilar, 25, referring to Mexico's patron saint.

Carrying banners and placards, members of dozens of Catholic organizations from the Mexico City area sang hymns as they marched and filled the plaza before the massive shrine as Cardinal Rivera celebrated Mass.

The Mexican bishops' conference issued a statement June 19 attributing recent violence to social problems that have gone "decades without being resolved" and urged the country "to work so that peace reigns."

Dozens were injured when striking teachers clashed with Oaxaca city police June 14. Analysts have noted that unions and other organizations affiliated with political parties can get rowdy around elections because they think that, in an effort to avoid violence, authorities will grant them more concessions.

Bishop Jose Martin Rabago of Leon said the conflict appeared to be politically motivated.

"These problems (involving the teachers) are rather politicized and are being used as leverage and to provoke some form of instability," Bishop Martin said, according to a June 16 report in El Universal newspaper.

-- CNS