LONDON (AP) _ Britain issued a formal protest Wednesday of South Africa's state of emergency and jailing of anti-apartheid activists, one day after the Conservative government reversed policy and met with a black guerrilla leader.

The Foreign Office summoned South African Ambassador Denis Worrall to receive the protest from Ewen Fergusson, deputy undersecretary.

In Pretoria, Tessa Solesby, second-ranking minister at the British Embassy, called on a senior official of the South African Foreign Ministry with a similar message.

The protest followed the meeting Tuesday between Lynda Chalker, a deputy to Foreign Secretary Sir Geoffrey Howe, and Oliver Tambo, head of the banned African National Congress, which is fighting a guerrilla war against South Africa's white authorities.

South Africa declared a nationwide state of emergency June 12. Foreign reporting groups say at least 3,000 people have been detained without charge, but President P.W. Botha's government has not released figures.

Prime Minister Margaret Thatcher previously had refused to meet with the ANC and, like the United States, has rejected the tough economic sanctions Tambo and other black leaders seek as a means of forcing an end to the apartheid policy of race descrimination.

Mrs. Chalker told the House of Commons on Wednesday that Mrs. Thatcher is going to the meeting of Common Market heads of government Thursday in the Netherlands with an ''open mind'' on South Africa, but will support only effective sanctions.

Her consistent position in the past has been that economic sanctions do not achieve their purpose.

The Rev. Allan Boesak, a cleric of mixed race who is a leading South African foe of apartheid, arrived in London on Wednesday. He said the state of emergency, which includes severe restrictions on what journalists may report, was an indication that the government was worried despite its ''violent powers.''

''We now know better than ever before the nature of the beast we are fighting,'' he said. ''I think for the first time the British government is beginning to understand the realities of the South Africa situation.''

Boesak, president of the World Alliance of Reformed Churches, held a news conference in the city hall at Brent in west London. The borough council invited him to take part in its current public education program about race segregation in South Africa.

He said he could not speak about economic sanctions or other key issues because he faces a charge of subversion at home.

''Leaving sanctions aside, the general feeling of black people is that Mrs. Thatcher, (President) Ronald Reagan, and (West German) Chancellor Helmut Kohl are the strongest supporters of the apartheid regime outside South Africa,'' Boesak said.

Asked what he thought the consequences of his visit would be and whether he would be allowed back into his country, he said: ''I do not know. I think they would rather have me there than here.''