Five days ago, Moscow cyberneticist Viktor Brailovsky had just been arrested and was sitting in a Soviet jail cell, belt and shoelaces taken by police, believing he had seen the last of his family for some time to come.

Today, Brailovsky stood amid a crowd of foreign scientists gathered in his living room and with a haunting smile thanked them for coming here to support Soviet Jewish scientists who have been refused permission to emigrate.

The wrenching contrast between apprehension and a sense of security characterizes both the current situation of Soviet Jewish activists and the unofficial seminar Brailovsky and four Moscow colleagues organized in hopes of strengthening their chances of eventually leaving this country with their families.

In the days before the unofficial seminars opening yesterday, KGB security police searched the apartments of Brailovsky and scientist Yuri Golfand, confiscating research papers and Hebrew-language materials and threatening prosecution for anti-Soviet slander.

Brailovsky, a slight, bearded father of two, was hauled off by agents last Thursday, dumped in a local police station cell and told he was a suspect in a new case against a samizdat journal, Jews in the U.S.S.R, which the has helped edit for several years. But as abruptly as the police arrived, the authorities changed their minds and freed him. He spent the rest of the week preparing for the seminar, the third such session held by Jewish scientists here since police broke up the first attempt in 1974.

Today, about 20 foreign scientists, including four American, met with an equal number of Soviet counterparts to deliver research papers, commiserate, and bear witness that the Soviets are not forgotten by the West.

While a number of foreign scientists were refused visas, those who received permission to come here took it as a sign, however murky, that the authorities retain an interest in continuing scientific contacts with the West despite the sharp U.S. and European reaction to the Soviet invasion of Afghanistan and the fact that Jewish emigration has declined in recent months.

Among the papers that have been delivered is one by exiled dissident leader Andrei Sakharov, and another by imprisoned human rights activist Yuri Orlov, who reportedly has suffered new, harsher treatment because he persists in attempting scientific research while in labor camp in the Urals.