A 37-year-old mathematician who was one of the organizers of an underground labor union here was sentenced last night to seven years in a labor camp and five years' internal exile, his friends reported today.

The judge ordered the maximum term after Valery Senderov was found guilty on seven counts of anti-Soviet slander and propaganda, including distribution last year of a leaflet calling on workers to stage a strike against the traditional subotnik, or working one Saturday free before Lenin's birthday.

In another case involving charges of anti-Soviet propaganda and slander, Ukrainian poetess Irina Ratushinskaya went on trial today in Kiev, according to dissident sources. Her poems have been circulating in the West and broadcast by the Voice of America. Some of her poems were openly hostile to the Soviet Union, referring to it as "my hateful motherland" in one poem.

Both trials appeared to foreshadow a series of legal actions against a number of dissidents arrested last fall in a major crackdown on religious and political activists.

Three other activists of SMOT, the suppressed underground union in which Senderov had been active, are scheduled to stand trial in the coming weeks. Also scheduled for trial soon are five young leftist socialists who contended that socialism does not exist in the Soviet Union and who embraced some ideas of Yugoslav dissident Marxist Milovan Djilas.

According to his friends, Senderov's one-day trial ended with a defiant statement by the activist.

"What we have seen here," Senderov was quoted as saying, "expresses the outrageous, cruel and hypocritical system in this country. If I am ever released from prison, I shall continue to fight it."

After the trial, other activists in SMOT, which is an acronym from the Russian words for the Free Interprofessional Association of Workers, passed a statement to Western journalists in which they asserted that they intended to continue their activities. The group, organized in 1978, now exists only in name although its organizers claim to enjoy the backing of 3,000 workers.

Since the turmoil in Poland, Soviet authorities have ruthlessly suppressed all manifestations of independent labor activities.

Although his friends said the issue was not mentioned during the trial, Senderov and another mathematician, Boris Kanevsky, wrote a study last year alleging that Jewish students were discriminated against at Moscow State University.