JOHANNESBURG, FEB. 11 -- Black activist Winnie Mandela and three codefendants pleaded not guilty today to charges of kidnapping and assault, but the trial was later delayed until Tuesday when the prosecutor told the court his first witness had been kidnapped.

Prosecutor Jan Swanepoel said he learned this morning that Pello Mekgwe, one of three key state witnesses, had been "taken against his will" Sunday from the premises of a Methodist church in the Johannesburg area. Swanepoel said he needed until Tuesday to prepare his second witness and was granted the delay by the Rand Supreme Court.

"This is a very serious development," he said. "It has an intimidating effect on other witnesses. I can't proceed when witnesses' lives are in danger."

Mekgwe was one of the four youths allegedly kidnapped and taken to the Mandela home in Soweto township, where they were said to have been assaulted the night of Dec. 29, 1988. One of the four, "Stompie" Mokhetsi Seipei, was taken from the house Jan. 1 and found murdered a few days later in a nearby field. A former Mandela bodyguard was found guilty of the murder last year.

News of the missing witness came today after Mandela, wife of African National Congress leader Nelson Mandela, and three codefendants each pleaded not guilty to four charges of kidnapping and four of assaulting the four youths.

Swanepoel, in his opening statement, said he would bring evidence of other kidnappings in which Winnie Mandela allegedly participated, and show that blood belonging to Mekgwe, the missing witness, was found on the walls, ceiling and curtains of a room in her house.

George Bizos, who leads the Mandela team of lawyers, read a statement signed by her in which she denied participating in any of the kidnappings or assaults.

According to her statement, she had been approached by Xoliswa Falati, now one of her codefendants, late in December 1988, and told that Falati was looking after a number of young men who were being sexually abused by a Methodist minister at a mission house.

Mandela said she had left town the same day as the alleged kidnapping of the four and when she returned two days later, Falati told her she had arranged for the four to stay at the Mandela home.