DJIBOUTI, April 11 (Reuters) - Djibouti has arrested and repatriated nearly 7,000 immigrants this year, mostly Somalis and Ethiopians, in a crackdown on illegal migration to the Red Sea state, police said on Friday.

Thousands of people in the impoverished Horn of Africa risk death every year trying to cross the shark-invested Red Sea in rickety boats from Djibouti to Yemen, seen as a gateway to wealthier parts of the Middle East and Europe.

Djibouti is one of the closest African countries to Yemen, and is safer than lawless Somalia for migrants from landlocked Ethiopia and the northern parts of Somalia.

"This period is the season to pass through the border because it is the cooler November-to-May season. Otherwise in the summer many immigrants die in the desert from thirst," Police Lieutenant Abdourahim Ali told Reuters.

The police said 6,723 immigrants mainly from Ethiopia and Somalia have been arrested and sent home this year. Djibouti caught and repatriated 16,091 people in 2007, and 12,579 in 2006.

Earlier this month, at least 53 Somalis drowned off the coast of Yemen while trying to cross from Somalia to the Arabian peninsula.

Near-daily violence in southern Somalia from an Islamist-led insurgency against the Somali government and its Ethiopian allies has displaced hundreds of thousands.

Djiboutian police said a recent surge in violence in Mogadishu had caused increased numbers of would-be migrants to come into Djibouti. (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/) (Reporting by Omar Hassan; Writing by Jack Kimball; Editing by Bryson Hull and Caroline Drees)

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