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The death toll from a suspected bomb attack on a bus in Guatemala City on Monday has risen to six, after one of those injured died in hospital.

A police spokesman said a woman had left the bomb on the bus and later detonated it by mobile phone.

The victims include a 35-year-old woman and her two sons, aged three and 11.

Mr Gonzalez said a faction of the Mara 18 gang was suspected of being behind the blast. Guatemala's public transport is subject to regular extortion.

Guatemalan police estimate that bus drivers paid out more than $1.5m (£970,000) in extortion money over the period to organised crime groups, who demand a "circulation tax" for those driving through their areas.

Local human rights groups say that between January and November 2010, 119 bus drivers and 51 other transport workers were murdered.

Gamaliel Chin, president of the association of transit operators, said it had asked the authorities for more protection after receiving four anonymous telephone calls in December threatening an attack.

Monday's attack happened while the bus travelled down a motorway connecting the centre of the capital with its north-western districts.

State prosecutor Rony Lopez told reporters on Tuesday that the suspected bomber, a 20-year-old member of a Mara 18 cell, had been arrested after being identified by witnesses, according to the Efe news agency.

She was detained in the western district of Colonia El Milagro, Mr Lopez added.

Guatemala has one of the highest murder rates in the world, with many of the killings blamed on gangs linked to Mexico's violent drug cartels.