Hundreds of thousands of believers, some walking for hours, paid homage to Mexico's most revered Catholic figure on Wednesday in a colourful, noisy show of faith that defied growing challenges to the church.

In one of the world's biggest annual pilgrimages, Mexicans sang and prayed in front of centuries-old cloak with the image of the Virgin of Guadalupe, who is said to have appeared to an Indian peasant in the 16th century.

Carrying elaborate flower wreaths, crowds of people flowed through a giant basilica in Mexico City to pass briefly on an airport-style moving walkway in front of an image of the dark-skinned virgin.

The church has become embroiled in a fight with the leftist government of the capital, which in less than a year has legalised gay unions and abortion and allowed terminally ill people to refuse treatment.

But religion is still a refuge for millions of mostly poor Mexicans.

"The government can pass whatever laws they want but the people will keep coming," said mechanic Antonio Chavez, 46, who walked over seven hours with his family from the small town of Acuitlapilco to ask the virgin for good health.

"You can't separate people from their faith," he said rubbing his sore legs. Chavez, like thousands of others, spent the night huddled under blankets outside the church waiting to sing to the virgin overnight.

Municipal authorities estimated that several million faithful have made the trip to the basilica in recent days. The annual Muslim haj to Mecca draws over 2 million visitors.

In November, a mob of leftists angered by church bells interrupting a political rally, burst into Mexico City's main cathedral, tearing down railings and overturning pews.

It was a sign of the growing tension between leftists and the Catholic hierarchy which opposes abortion and gay unions.

Mexico is the second-largest Catholic country in the world but church doctrine on social and family matters is often taken with a pinch of salt.

"The Catholic hierarchy has lost touch with popular Catholics who get divorced and have abortions despite what the Church says," said Elio Masferrer, a religion expert at Mexico's National School of Anthropology and History.

The Virgin of Guadalupe is said to have appeared to peasant Juan Diego in 1531 on a hilltop which had once been a shrine to an ancient Aztec goddess.

When Juan Diego told a bishop of his vision, the virgin's image appeared on his cloak. That event was key in converting Mexico to Catholicism.

The humble woodcutter was canonised by Pope John Paul II in 2002 in an effort by the Vatican to reach out to Latin American believers.