Soviet authorities prevented dissident biologist Sergei Kovalyov from taking a job at a research institute only a day after he spoke against human rights abuses during a meeting with President Reagan in Moscow, activist leader Andrei Sakharov said Friday.

Speaking at a news conference, Sakharov, a Nobel Peace Prize winning physicist who was exiled for seven years because of his own dissident activity, said the victimization of Kovalyov was a direct challenge to Reagan.He also said there were at least 20 political prisoners still in jail and exile in the Soviet Union and he demanded their immediate release as a show of good faith from Soviet leader Mikhail Gorbachev.

Sakharov said Kovalyov, one of 49 dissidents and refuseniks invited personally by the president to attend a luncheon at the U.S. ambassador's residence in Moscow last Monday, was informed 24 hours later that the job he had been given at the Institute of Problems of Technical Transfers was no longer available.

"The next day after that meeting with President Reagan the papers were withdrawn," Sakharov said.

"Authorities warned the institute could be hampered if the man was given a job at the institute. This is a revival of the past," Sakharov said.

"It is a direct challenge to those who organized the meeting," he said.

Kovalyov was fired from his job at another scientific institute when he began to participate in the human rights dissident movement in the 1970s. He was the editor of the Chronicle of Current Events, which for years detailed human rights abuses by Soviet authorities. It took years for the KGB under Yuri Andropov to finally extinguish the journal - arresting all its contributors and editors.

Sakharov said Kovalyov had spent 10 years in exile between 1974 and 1984 for his dissident activities.

The 90-minute news conference by Sakharov was held at the Soviet Foreign Ministry. Sakharov, who has become increasingly acceptable in official circles, said he was actually approached by the ministry and asked if he would hold the news conference because of numerous requests from foreign journalists covering the Reagan-Gorbachev summit.

Sakharov, who now works for the Soviet Academy of Sciences, said he knows of 20 "prisoners of conscience" either in jail or in exile for their religious or political beliefs. He said the Kremlin has it own list which has only 13 names but includes only those in jail, not in exile.

Sakharov also called for a complete overhaul of the Soviet penal system and the elimination of laws making political activites a crime.