In the second major release of political prisoners in three days, the South African government today freed 11 black leaders who had been detained without charges for more than three months.

It immediately charged six of them with high treason, refused them bail and returned them to prison.

The six, all leaders of the country's leading black political organization, the United Democratic Front, were detained in August, apparently for helping to organize a boycott of elections for the mixed-race and Asian communities held under a new national constitution.

Conviction on a charge of high treason carries the death penalty in South Africa.

The decision to release and promptly charge some of the black leaders strengthened an impression gaining ground in political circles here that as the Pretoria government responds to U.S. pressure to end detention without charges, it is planning to stage a series of trials instead.

Of 16 political prisoners released Friday, five were charged with "subversion," apparently for organizing a two-day protest strike by black workers last month. South Africa's stringent security laws make striking a subversive crime, punishable by 20 years' imprisonment, if it interrupts for political ends industrial activity or the production and distribution of goods.

Those five, the organizing committee of the protest strike, were each released on $1,000 bail.

The releases that began Friday, the day President Reagan met Nobel Peace Prize laureate Bishop Desmond M. Tutu, were seen here as an attempt by the Pretoria government to defuse the growing campaign in Washington against the administration's policy of "constructive engagement" with South Africa.

News of the first 11 releases was announced by the South African Embassy in Washington hours before there was any announcement here, and President Reagan immediately claimed credit, saying it was the result of his administration's "quiet diplomacy" with Pretoria.

In Johannesburg today, the democratic front described the release of the detainees as an act "calculated to accord credibility to the constructive engagement policy of the Reagan administration."

The 11 released Friday night included the country's two top labor leaders, Christopher Dlamini, president of the 120,000-member Federation of South African Trade Unions, and Piroshaw Camay, general secretary of the 150,000-member Council of Unions. They have not been charged.

Those released today included three United Democratic Front leaders who took refuge in the British Consulate in Durban in September, when detention orders were issued against them, and who were arrested when they left the consulate building Oct. 5.

The three, George Sewpersadh, 48, Mewa Ramgobin, 52, and Mooragiah Naido, 53, are among those who have been charged with treason.

Three others who sought refuge in the consulate with them, and are still there, were consulting with their lawyers tonight before deciding whether to end their 89-day hideout in the consulate.

The detention orders against them have been withdrawn along with the others, but it seems virtually certain that if they emerge from their diplomatic sanctuary they, too, will be arrested and charged with treason. Members of the security police were waiting outside the building tonight.

With the withdrawal of the detention notices, Britain stepped up its pressure on the three to leave. A statement issued by the Foreign Office in London said, "The situation has now changed fundamentally, and we expect the three to leave at once."

The three are the United Democratic Front's president, Archie Gumede, Billy Nair and Paul David.

Among those released today and not charged were the democratic front's national secretary, Popo Molefe, and its publicity secretary, Mosiuoa Lekota.

Muntu Myeza, general secretary of a rival black political movement, the Azanian People's Organization, was also released without charges.

Molefe told a press conference in Johannesburg that he was subjected to "ruthless and cruel" treatment while in detention.

He said he developed a lung infection and was taken handcuffed and in leg irons to a hospital in a police van that had no windows.

"I was tied to the bed with handcuffs after being admitted and guarded by three warders who worked in shifts. They even took me to the toilet," Molefe said.

According to the Detainees' Parents Support Committee, an organization that monitors the detention system closely, there are still 207 political prisoners being beld without charges.

They include six black student leaders who were arrested in Port Elizabeth, in eastern Cape Province, Friday, a few hours before Reagan claimed the release of the other 11 as a victory for U.S. diplomacy.