The number of people arrested in South Korea for criticizing the government has increased in the past two years following growing protests by students, workers, farmers and other groups, according to a report released yesterday by Amnesty International.

The report also describes the continuing use of torture on some prisoners and provides additional detail beyond the State Department's report on human rights, which was issued earlier this year. The State Department said reports of torture or cruel treatment of prisoners in South Korea increased "significantly" in 1985 over 1984.

The report from the London-based human rights organization, comes at a time when the South Korean government is under increasing pressure, both at home and abroad, to appear flexible and reasonable in dealing with dissent as the country prepares for a transfer of power in 1988, when President Chun Doo Hwan has promised to step down, and as it prepares to host the Olympic Games, also in 1988.

According to Amnesty, in the past two years, the Seoul government has increased its use of short-term detention -- up to 29 days in jail -- for those charged with taking part in demonstrations likely to cause "social unrest" or "spreading groundless rumors." Others have been given long prison terms on charges of being "procommunist," endangering national security or spying for North Korea, after trials lacking fundamental safeguards of fairness, according to the report.

In one case, two high school teachers were sentenced in February to 18 months in jail in connection with the publication of a magazine that criticized the government's education policies. The charges against them included criticizing the education system from a "radical" perspective.

The methods of torture detailed in the report include electric shock, beatings, and deprivation of food and sleep.

Amnesty charged that the three main intelligence agencies, the Anti-Communist Bureau of the National Police, the Agency for National Security Planning and the Army Security Command, have been regularly reported to have tortured and mistreated political prisoners.

Amnesty's report is based on two visits to South Korea in 1984 and 1985 and on "continuous monitoring of information from that country," it states. The information covers events through March of this year.

Amnesty officials said the group sent the Seoul government a memorandum urging a halt to the abuses. The government has not responded to specific allegations, Amnesty said.

Amnesty's report describes cases in which prisoners told the courts trying them that their confessions were false and only the result of torture and that they were convicted on the basis of those confessions. Many were held incommunicado and denied access to lawyers during the pretrial period when they should have been preparing their defense.

In one case, Kim Keun Tae, 39, the former chairman of a student opposition organization, was sentenced to seven years in jail in March on charges of having organized or participated in meetings and demonstrations "feared to cause social unrest" and of promoting North Korean propaganda. At the start of his trial in December, he testified that he was tortured by the Anti-Communist Bureau of the National Police during his interrogation in September.

"They would put me on a table with a blanket under me and tie my heels, knees, thighs, stomach and chest to the table," he testified in Seoul District Court. "They poured water on my head, chest and groin to help conduct the electricity into my body. Then came the electric shocks, at first, light and short, then stronger and longer. The electric shock brought me within the shadow of death . . . ."