LONDON (AP) _ Tens of thousands of demonstrators chanting ''Free Nelson Mandela 3/8'' thronged Hyde Park on Sunday to mark the 70th birthday of the jailed South African leader. It was one of the biggest anti-apartheid rallies ever in London.

Against a backdrop of flags of the African National Congress, South African Archbishop Desmond Tutu led the rally in chants that echoed across central London.

''In 25 years in jail, he (Mandela) has not spoken to the public, yet many people not born then have been inspired by him,'' declared Tutu.

''This man is head and shoulders above any contemporary leader in South Africa,'' Tutu said. ''Any sensible government would realize that Nelson Mandela is absolutely essential for peace and stability in southern Africa.''

Mandela turns 70 on Monday. He was jailed in 1962 for leaving South Africa illegally and inciting unrest. While serving a five-year sentence, he was given a life term in 1964 for sabotage and plotting the overthrow of the government.

South Africa's white-led government banned all major birthday celebrations for Mandela, whose ANC guerrilla movement is outlawed there.

Organizers arranged a last-minute concert Sunday at the University of Cape Town, but police stopped it about two hours after it began. The audience of 500 left peacefully.

The organizers then announced two events for Monday - a church service in Cape Town and a news conference in Johannesburg that is expected to include members of the Mandela family. But police refused to grant permission for outdoor gatherings and none was scheduled.

Blacks were expected to boycott schools in some areas Monday.

Dozens of foreign governments called for Mandela's release in recent days.

Soviet leader Mikhail S. Gorbachev told Mandela, ''It distresses one to realize that you mark the birthday in a prison dungeon into which you were thrown by the racists more than a quarter of a century ago.''

''Soviet people firmly demand your release, Comrade Mandela, and the release of all political prisoners in the Republic of South Africa,'' Gorbachev said in a telegram Sunday, which was published by the official Tass news agency. ''I give you a strong handshake.''

In Italy, thousands jammed piazzas for a Saturday night concert in Bologna and another one Sunday in Rome. Dutch and Swedish anti-apartheid movements scheduled events for Monday.

In central London, traffic was snarled as the demonstrators, headed by 25 activists who had marched 500 miles from Glasgow, Scotland, poured into Hyde Park under rainy skies.

The organizers, Britain's Anti-Apartheid Movement, estimated the crowd at 250,000 people, but police put the figure at only 30,000. Scotland Yard reported three arrests on minor offenses.

The mostly young, white crowd swayed as a South African band belted out African nationalist themes from the podium, which was ringed with red and black tribal warrior shields.

They roared with approval as the face of Winnie Mandela, the jailed leader's wife, flashed on a giant video screen. In a message taped in South Africa, she said her husband's birthday ''relates to a symbol of resistance in this country.''

''We are very moved, very touched, by this gesture,'' she said, expressing thanks ''to the entire international community'' for applying pressure for her husband's release.

Security forces, some in armored vehicles, patrolled South Africa's major black townships Sunday, although there were no reports of violence. Police also set up several roadblocks, including one outside Cape Town's Pollsmoor Prison where Mandela is held.

Six other purple-robed bishops, in Britain for the Lambeth Conference of Anglican bishops held every 10 years, joined Tutu on the podium.

They sat with labor union activists, opposition Labor Party members of Parliament, Commonwealth Secretary General Sir Shridath Ramphal and Sir Richard Attenborough, whose film on South Africa, ''Cry Freedom,'' has attracted huge audiences.

The crowd cheered as speakers denounced Western leaders, particularly British Prime Minister Margaret Thatcher, for refusing to impose tough economic sanctions against South Africa.

Mandela has refused an offer of release by the South African government on the condition he renounces violence, saying that the real prepetrators of violence are South African authorities.

South African diplomat Justus de Goede, the second-ranking official at the London Embassy, said in a radio interview the ANC was committed to the violent overthrow of the government, and noted that bomb and grenade attacks by ANC guerrillas were increasing.

''I think it is asking a great deal of any government to knowingly release the leader of an organization which has this political manifesto into society without some sort of guarantee,'' de Goede said.

The ANC is the main guerrilla group fighting South Africa's government and its system of apartheid, under which the country's 26 million blacks have no voice in national affairs. The 5 million whites control the economy and maintain separate schools and districts.

In addition to Gorbachev, West German Foreign Minister Hans-Dietrich Genscher, Malaysian Prime Minister Mahathir Mohamad and Danish Foreign Minister Uffe Ellemann-Jensen all called Sunday for Mandela's release.