The government of the Comoros Islands in the Indian Ocean has sent troops to Anjouan island, to end a year-old rebellion.
Mortar and machine-gun fire were heard when the troops landed by sea early in the morning.
The soldiers seized the island's airport, before marching on the main city Mutsamudu.
A number of residents fled the fighting as the invading force closed in on the city.
African Union forces are supporting the operation to remove a colonel whose election as Anjouan leader last year is regarded as illegal by the Comoros government.
Troops loyal to renegade Colonel Mohamed Bacar were ousted from Mutsamudu, but sporadic fighting continued in other parts of Anjouan. The colonel's whereabouts are not known.
Tanzanian soldiers captured two men they say were fighting for Colonel Bacar.
Leaflets were dropped on Anjouan by the Comoran army ahead of the assault, warning residents that it was imminent.