Sixteen blacks were given jail terms today ranging from five to 17 years for their underground activites on behalf of the outlawed black movement, the Pan-Africanist Congress, further crippling an organization that once was a formidable political force in South Africa.

Today's sentences follow another setback to the movement - the assassination June 12 of the movement's United Nations representative, David Sibeko. He was apparently the victim of intraparty squabbling that has plagued the Congress since its first president Robert Sobukwe, died of cancer of February 1978.

In the 1960s, during widespread civil disobedience by blacks against South Africa's policy of racial discrimination Sobukwe's Congress persuaded thousands of blacks to invite arrest by appearing at police stations throughout South Africa without the identification papers they were required to have with them at all times.

At one police station in Sharpeville, 69 blacks were shot dead by police, touching off a period of unrest in black communities and a government crackdown that ended in the banning of the Congress and the other black nationlist movement from which it had split, the African National Congress.

Sobukwe was jailed until 1969. He was then placed under a banning order that barred political activity and communication with more than one person at a time.

Sobukwe's death set off an internal fight for control. Potlake Leballo was eventually elected president, but he has not been able to unify the movement, as evidenced two weeks ago by Sibeko's murder.

Dissident members of the organization have been arrested in Dar es Salaam, Tanzania, where Sibeko was killed.

The conviction of Congress members is regarded as a further blow to the Congress. The large number of defendants and the amount of time and money the state has invested in the trial was interpreted by some analysts as a government attempt to stamp out the Congress' clandestine internal organization.

The man reputed to be the Congress's leader in South Africa, Zephania Mothopeng, 65, received a 15-year sentence, effectively consigning him to prison for the rest of his life.

Along with Mothopeng, 15 other men were found guilty of "reviving" a banned organization and of sending youths out of the country for training in guerrilla tactics. Some were also found guilty of inciting riots.

These activities carry a maximum penalty of death. There is no parole in political crimes.

The trial was held in the small, rural town of Bethal, a two-hour drive from Johannesburg. This is a common practice in security trials cut down on attendance by supporters of the defendants and by the press.

Passing sentence, Supreme Court Judge D. Curlewis said that the older men had corrupted the youths, encouraging them to undergo military training abroad. For this, they "were wicked men and what they did was wicked," he said.