LUSAKA, Zambia (AP) _ Anti-apartheid guerrillas and their supporters in the Organization of African Unity on Wednesday condemned President Reagan's speech on South Africa, called on Congress to impose sanctions and promised escalated warfare.

The Zambia-based African National Congress, the main black South African guerrilla group, called Reagan hypocritical. It issued a communique contending Reagan was not genuinely interested in ending apartheid, the system of racical segregation under which South Africa's 5 million whites dominate the country's 24 million blacks.

In his speech Tuesday night, Reagan called on the white-led South African government to release political prisoners and establish a timetable for ending apartheid. He rejected economic sanctions as a means of bringing about change and attacked the ANC for what he termed terrorist acts.

The ANC statement hailed the statements by some U.S. congressmen that they would propose legislation providing for punitive economic measures against South Africa.

The ANC said it had ''no choice but to ensure the all-round intensification of the struggle until apartheid is destroyed and South Africa transformed into a united, democratic and non-racial country.''

In the Ethiopian capital of Addis Ababa, the Organization of African Unity also appealed to Congress to override Reagan and impose sanctions through legislation.

The organization's Council of Foreign Ministers, meeting to set an agenda for the July 28-30 summit in Addis Ababa, interrupted its work Wednesday morning for a special session to discuss Reagan's speech.

A leader of The Pan Africanist Congress, another guerrilla group fighting the Pretoria government, condemned Reagan's speech at a news conference in Addis Ababa. Ahmed Gora Ebrahim, denounced the official position of the governments of the United States, Britain and West Germany: that sanctions would worsen the already dire lot of South African blacks.

''Would they also today oppose the abolition of the evil institution of slavery on the grounds that it would lead to unemployment of the liberated slaves?'' Ebrahim asked.

Nigerian Gen. Josephy Garba, chairman of the U.N. Anti-apartheid Committee, said Reagan's speech sounded as if it were written by South Africa's president, P.W. Botha.

''Africa should do what it should have done in the first place, which is intensify the armed struggle,'' Garba told reporters in Addis Ababa. ''We must now realize the time for negotiation, the time for peaceful settlement is gone and I don't think it will ever come back.''

Foreign Minister Bolaji Akinyemi of Nigeria made a similar appeal.

''The best reply from Africans to Reagan's speech is to increase support to liberation movements,'' Akinyemi said. ''Guerrilla war is the only solution.''