The chief administrator of the U.N. peace-keeping mission in Namibia arrived Friday to supervise the start at dawn Saturday of an independence plan for Africa's last colony and an end to a century of German and South African rule.

Thousands of blacks turned out to greet U.N. administrator Martti Ahtisaari as the final countdown to independence began but the biggest contender for control of the new nation, the South West Africa People's Organization, kept supporters away fearing violence.Buoyed by reports British Prime Minister Margaret Thatcher would jet into Namibia Saturday to visit British troops in the U.N. peace-keeping force and meet South African Foreign Minister Roelof "Pik" Botha, South African officials prepared for the first major steps in the independence plan.

Hours before it was to be implemented, Angola, Cuba and South Africa swapped 16 political prisoners in a long-awaited exchange signaling a U.S.-brokered agreement between the three countries on South African acceptance of Namibian independence in exchange for a withdrawal of some 50,000 Cuban troops from Angola.

At dawn Saturday, a formal cease-fire between South African troops and SWAPO guerrillas was to take effect, following the confinement of an estimated 25,000 South African Defense Force troops and 18,000 Namibian security forces to their bases.

"It is the eve on an era," said Ahtisaari, who has been waiting for a full decade as U.N. Nambiain representative to implement Security Council Resolution 435 guiding Namibia to independence.

"It will be the era of all Namibian people," he said.