GUATEMALA CITY (Reuters) - Guatemala has banned motorcyclists from carrying passengers in an attempt to crack down on drive-by murders by gunmen perched on the back of moving bikes.

The Central American country is terrorized by violent street gangs who rob car drivers at gunpoint and kill bus drivers, among others, for failing to pay extortion fees. Hitmen often shoot victims from the back of a motorcycle, which lets them make a speedy getaway if traffic is heavy.

The law implemented this month also requires motorcyclists to wear brightly colored jackets and helmets clearly marked with their vehicle registration number.

Flouting the law will mean a fine of up to $3,000. Police will start charging in May to give people time to comply.

"We're still seeing many bikes with two people. We make the passengers get off but we can't yet give out fines," police spokesman Marco Trejo told Reuters on Monday.

Authorities are struggling to contain youth gangs like the "Mara 18" and "Mara Salvatrucha," which have thousands of members from the United States to Central America and live off extortion, armed assault and drug dealing.

The country of 13 million people is also plagued by crime linked to Mexican drug cartels smuggling South American cocaine north.

More than 40 bus drivers have been killed this year in Guatemala City, many in bike attacks, for not paying off the gangs in an extortion racket worth an estimated $10,000 a day.

The signature technique has also been used recently to murder prominent Guatemalans including a television reporter and the former director of the national chamber of industry.

With over 6,000 murders last year, Guatemala is one of the most violent countries in Latin America.

The ban on pillion passengers is irking many motorbike drivers. "I use my motorcycle to take my children to school and my wife to work," courier Jorge Monterroso said.

"It's too dangerous for them to use the buses where all kinds of people can get on, take out a gun and start shooting."

(Reporting by Sarah Grainger, editing by Patricia Zengerle)