(Adds details, pirates)

By Abdiqani Hassan

BOSASSO, Feb 6 (Reuters) - Somali gravediggers on Wednesday buried 20 people, most of them Ethiopians, killed in an explosion in the northern Somali port of Bosasso, and police tried to establish the cause of the blast.

At least 100 others were also wounded in Tuesday's explosion inside a small cinema popular with migrants from Ethiopia -- the ancient rival of Somalia.

Ethiopians often come to Bosasso from their landlocked nation, hoping to make the dangerous trip across the Gulf of Aden to Yemen in search of stability and a better life.

Thousands of people fleeing poverty and violence in Somalia and Ethiopia make the trip annually, but hundreds perish each year when often overloaded boats capsize or sink in the shark-infested waters.

Hundreds of people including elders and government officials attended the burial in the semi-autonomous region of Puntland, ordinarily one of the more stable parts of the anarchic Horn of Africa nation -- but not immune to violence.

"We buried all the dead in three mass graves after their fellow Ethiopians in Bosasso arrived and identified their loved ones this morning," Bosasso Mayor Abdirisaq Hared told Reuters.

Puntland is peaceful compared with southern Somalia, where the interim government and its Ethiopian military allies are battling an insurgency led by Islamist militants.

Last week, gunmen hijacked a Russian ship off the coast of Somalia and were holding hostage the four Russian crew members, an Irish chief engineer and a British captain aboard the Svitzer Korsakov.

"The gunmen holding captive the Russian built tug boat are known as the Ocean Salvation Corps. They say they are eco-warriors but not pirates," said Andrew Mwangura, director of the Seafarers Assistance Programme.

Mwangura said there were several vigilante groups patrolling off of Somalia where they police the waters "fining" boats on charges such as environmental damage or fishing without permits.

Piracy has been rife in the waters off Somalia since warlords toppled military dictator Mohamed Siad Barre in 1991, and attacks have reached unprecedented levels due to instability onshore as the interim government battles insurgents. (Additional reporting by Wangui Kanina; Writing by Aweys Yusuf; Editing by Daniel Wallis and Jon Boyle)

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