Hundreds of thousands of black workers and students remained on strike Tuesday to demand the right to oppose apartheid. Police said seven people were killed and more than 20 injured.

Strike supporters and transport companies said the protest, in its second day, eased in many areas and worker attendance increased. But participation near 100 percent was reported in others.Police reported more than two dozen incidents of violence between Monday evening and dawn Tuesday, including many firebombings and stonings of homes, buses and trains. Twenty-one arrests were reported.

One black youth was killed by police shotgun fire after municipal officers were stoned, police said. The other six blacks who died, including three children, were killed in shootings and a hand-grenade attack in which the assailants were not identified.

In the black homeland of KwaZulu, hospital officials said a bus passenger injured in a firebomb attack early Monday had died.

A bomb caused minor damage to a rail line in Soweto early Tuesday, delaying some trains carrying blacks from the huge black township to jobs in Johannesburg.

Black union leaders had called for a three-day "national protest" without specifying a strike. They said the action aims to pressure business leaders into demanding that the white minority government ease restrictions on the anti-apartheid movement.

A three-day strike would be the longest nationwide protest since the government decreed an emergency June 12, 1986, to thwart a black revolt against apartheid, the policy of racial separation that reserves power as well as the best schooling and living for South Africa's 5 million whites and denies the country's 26 million blacks a voice in national affairs.