UPDATE 4-Calm Guinea-Bissau votes to replace slain leader

* No reports of trouble, turnout low

* Three front-runners among 11 contenders

* Second round might be needed (Adds comment from electoral commission)

By Alberto Dabo

BISSAU, June 28 (Reuters) - Guinea-Bissau's voters cast ballots in a quiet and orderly election on Sunday to replace the slain president of the West African country, hoping for a chance to end turmoil worsened by army rivalries and drug gangs.

But turnout was low and the electoral commission blamed tension after the killing of President Joao Bernardo Vieira on March 2, apparently in revenge for the assassination of the head of the army. One candidate was also killed during the campaign.

There were no immediate reports of trouble from across the former Portuguese colony of 1.6 million people, although heavy rains also discouraged voters in some parts of the swampy country.

"This is a great day. I'm voting for peace, calm and stability," said trader Binta Diallo in Bissau, the run-down coastal capital. "I want a president able to bring peace and quiet and to end the criminality."

The vote is also a test for a region worried at the retreat of democracy after coups in Guinea and Mauritania and a deepening political crisis in Niger.