(Fixes typo in first paragraph) (Updates death toll, adds details)

TEGUCIGALPA, May 3 (Reuters) - Police have quelled a riot at Honduras' main prison in which at least 18 inmates died, a Security Ministry spokesman said on Saturday.

The riot at the National Penitentiary north of the capital began late on Friday and was the second such incident in the country's overcrowded prison system in a week.

"The inmates were killed with knives and machetes ... . They were all covered in blood and there are several beheaded bodies," spokesman Hector Ivan Mejia told Reuters.

Last Saturday nine prisoners were killed in a riot at another prison in the northern city of San Pedro Sula.

The Central American country's 25 prisons are overflowing with some 12,000 inmates. Fights are frequent as members of street gangs, known as maras, vie for control inside the penitentiaries.

The streets of Honduras' main cities are blighted by maras, who often are involved in drug and weapons trafficking. (Reporting by Gustavo Palencia; Writing by Cyntia Barrera Diaz; Editing by Xavier Briand)

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