Hundreds of thousands of people have thronged the streets of Manila to fling themselves at a statue of Jesus Christ as it inched its way through the Philippine capital in an annual procession that is one of the world's biggest shows of Catholic devotion.

The faithful gathered before dawn on Wednesday to catch a glimpse of the statue as it was wheeled on a metal float along a seven-kilometre route through the city.

They believe touching the religious icon known as the Black Nazarene, or simply being in its presence, can heal the sick or deliver good fortune.

Police said at least 800,000 people were in the crowd.

"I survived a stroke because of him (God)," 70-year-old Joaquin Bordado, who has attended the procession for decades, told the AFP news agency. "I will do this every year until I am 100 years old."

Around him, the crowd, mostly walking barefoot as a sign of penitence, chanted "Viva Nazareno" (Long live Nazarene) and jostled for a glimpse of, or selfie with, the statue cloaked in a maroon robe that is topped with a crown of thorns and cross.

Many of them, in yellow and maroon shirts, threw white towels to people on the carriage to wipe on the statue.

'Rhythm of peace'

The charred statue is believed to have survived a fire in the 17th century while on its way to the Philippines, which became Asia's bastion of Catholicism during 400 years as a Spanish colony.

The Red Cross reported more than 100 injuries including cuts and dizziness in the first few hours of the procession. Every year, hundreds are hurt and some deaths are not unusual.

"It is so dangerous to join the procession. If you see people charging forward, this makes me nervous," said 21-year-old college student Angelica Alcantara.

"Many young people do this for fun but this is about your faith in God," she added.

Critics say the procession, which usually takes about 20 hours, is a mixture of superstition and unnecessary risk for participants.

But Church officials say the practice is a vibrant expression of faith in an overwhelmingly Christian nation of 105 million.

"If you are an outsider, you will not hear, see or feel that faith. You will only see a very unruly or chaotic situation," said Father Danichi Hui, a priest at the procession's destination, Quiapo church.

"But inside there is a rhythm of peace. There is a serenity," he said.