Bogota is used to mayoral proclamations

The move comes as a report by the UN children's fund Unicef calls for a "zero tolerance" policy to combat the multi-billion-dollar child sex industry which it says exploits millions of young people across the world.

"The sexual exploitation of children is perhaps one of the most deplorable customs that exists. What is of concern is that adults have a lot of opportunities to have sex freely among equals, but many minors are forced into it," Bogota Mayor Antanas Mockus said.

He said the curfew, which is timed to coincide with seasonal festivities, aims to reduce violence and drug abuse, and other crimes committed both by and against young people.

Worldwide problem

The Unicef report documents the extent of child prostitution across the world.

Unicef says millions of children are exploited for sex

It says there are 400,000 to 500,000 child prostitutes in India alone, while one third of Thailand's prostitutes are minors and the sex industry earns 10-14% of the country's gross domestic product.

Kul Gautam, a deputy executive director at Unicef, said that the trafficking of children for sex often uses the same channels as the drugs trade.

Colombia is notorious for its drugs trade, producing more than 80% of the world's cocaine and supplying more heroin to the US than any other country.

The BBC's Jeremy McDermott in Bogota says that as the curfew came into force, police fanned out to trawl Bogota's nightspots, but most of the children disappeared from sight as soon as the police vans arrived.

Controversial policies

In response to criticisms of the curfew, Mr Mockus said, "I have received an avalanche of accusations saying these and other measures restrict people's freedoms, but I would prefer people to look on this from a different, more positive perspective."

Our correspondent says the citizens of Bogota have ceased to be surprised by the edicts of Mr Mockus, who got married in a circus tent filled with tigers and strutted around the capital in a super hero outfit calling himself 'super citizen'.

Earlier this year he introduced a "women-only" night with a voluntary curfew for men, designed to encourage traditionally macho Colombian men to do the domestic chores and let women go out for an evening without male harrassment.

The night was a success, unlike a similar event without women, which went down less well with the city's residents.