BISSAU (Reuters) - Guinea-Bissau swore in the head of its National Assembly as interim head of state on Tuesday, a day after the assassination of President Joao Bernardo Vieira threw the tiny West African country into turmoil.

Vieira was killed in his home on Monday in an apparent revenge attack for the death on Sunday of a rival, armed forces chief General Batista Tagme Na Wai, ending a longstanding and bitter dispute that fueled years of violent conflict.

I swear on my honor to defend the constitution, laws, independence and national unity, and dedicate my intelligence and my energies to the service of the people of Guinea-Bissau, Pereira said at the ceremony at the National Assembly building.

The ceremony was attended by envoys from West Africa and Portuguese-speaking nations, including Angola, Sao Tome and Principe and Cape Verde, who flew to the capital Bissau on Tuesday in a bid to avert a possible coup or further unrest.

The army has denied any wish to seize power but soldiers guarded strategic locations in the city, raising questions as to who controlled the poor former Portuguese colony of 1.6 million, where drug traffickers have fueled years of instability.

The African Union's Peace and Security Council decided not to suspend Bissau as the attacks did not represent a coup. Neighboring Guinea was suspended from the AU after a coup in December following the death of its president.

The African Union appeals urgently to the political parties and actors of this country to exercise restraint and refrain from plunging the country once again into a spiral of power struggle, the continental body said in a statement earlier.

It said it would send an envoy to Bissau.

Senior envoys from Portuguese-speaking countries, including Portuguese State Secretary for Foreign Relations and Cooperation Joao Gomes Cravinho, arrived in Bissau on Tuesday.

We maintain constant telephone contact, but actually being there sends a different kind of a signal and gives another opportunity to talk. At this moment there is no indication of a need for any international or military force for Guinea-Bissau, Cravinho said on Portugal's SIC television before he set off.

UNCERTAIN CALM

Life in Bissau began to return to normal with some shops reopening on Tuesday, but people remained on edge. Shoppers briefly fled two market places in panic on Tuesday afternoon after rumors of fresh shooting, which appeared to be untrue.

Local radio stations resumed broadcasts. The army had ordered them to stop broadcasting on Sunday after Na Wai was killed in an explosion at the military headquarters.

Parliamentary Speaker Pereira becomes president for a limited period pending elections under the constitution, which the armed forces have promised to respect.

We have come here to reiterate to the government that this is no coup d'etat and that is not the intention of the military, Frigate Captain Jose Zamora Induta, the deputy head of the Navy and spokesman for the Armed Forces Officers' Commission, said on Portugal's RDP Africa radio late on Monday.

The twin killings have removed two of the most powerful figures in Bissau's recent history. The country has suffered repeated bouts of civil unrest, military revolt and coups since winning independence in 1974 after a bloody conflict.

Vieira, a guerrilla commander in the independence war, seized power in a coup in 1980. He was deposed by a military junta that included Na Wai in 1999 following a brief civil war, and was elected back into power in 2005.

Vieira's wife took refuge in the Bissau embassy of fellow former Portuguese colony Angola, and Angolan public radio reported that she had requested asylum in Portugal.

(Additional reporting by Tsegaye Tadesse in Addis Ababa, Henrique Almeida in Luanda, David Lewis in Dakar and Andrei Khalip in Lisbon; writing by Alistair Thomson; editing by Ralph Boulton)

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