By Pedro Uchi

MANILA (Reuters) - Three hundred volunteers in diapers were on hand on Friday to form human barricades during a jam-packed open-air Catholic Mass in Manila before a statue of Jesus Christ was paraded through city streets for the annual "Black Nazarene" procession.

More than five million Filipinos attended Mass at a park, with the Metro Manila Development Authority assigning adult diapers to volunteers who could not leave their posts, in a trial days before a visit by Pope Francis.

Emergency and police officials said they expect a much bigger crowd when the leader of 1.2 billion Roman Catholics celebrates Mass at the same venue on Jan. 18.

Authority chairman Francis Tolentino said only the volunteers at Friday's Mass were required to wear the adult diapers.

"About 300 of them because they cannot leave their positions otherwise people will come in and break the line," he said.

About 2,000 diapers were distributed to civilian auxiliaries on traffic duty for the procession, but there was no data on how many used them. In a radio interview, Tolentino said defaulters would not be penalised as the exercise was optional.

"I feel so uncomfortable wearing it," a man on traffic duty told Reuters when asked why he did not wear the diaper.

The experiment invited much ridicule on social media with critics questioning why the city authority did not rent more portable toilets. Many devotees were seen urinating in the park.

"This has got to be the dumbest idea of the year," said Twitter user Joseph Brian Calimon.

An authority official defended the experiment as a "practical" option, adding in a statement that adult diapers are "used regularly as standard operating gear" by U.S. soldiers, Buckingham Palace guards, and astronauts.

After the Mass, about a million people took part in the procession for a 5 km (3 mile) walk to the Basilica of the Black Nazarene with a centuries-old black statue of Jesus Christ, which is widely believed to have healing powers.

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Barefoot devotees lined the streets to see and try to touch the life-size statue of Jesus kneeling with a cross, in a festival held in the former Spanish colony for more than 200 years.

At least one devotee was crushed to death, while hundreds fainted or suffered minor injuries, as a sea of humanity was funnelled through narrow streets for the largest parade in the predominantly Roman Catholic country.

(Additional reporting by Manuel Mogato; Writing by Tony Tharakan; Editing by Robert Birsel)