ZAIRIAN TROOPS TAKE UP ARMS FOR REBEL CAUSE.

Byline: Howard W. French The New York Times

In the most dramatic setback for the Zairian government in a calamitous six-month civil war, troops in the country's second-largest city laid down their arms Monday and declared common cause with rebels advancing on the city.

With the rebels reported less than 20 miles west of the city, Lubumbashi, deserting soldiers of the 21st Brigade donned white scarves in a sign of surrender and declared that they would fight any government forces who attempted to stop the rebels.

Reports reaching the capital, Kinshasa, from Lubumbashi spoke of thousands of citizens turning out in the streets to hail the rebel leader, Laurent Kabila, who commands the Alliance of Democratic Forces for the Liberation of the Congo.

By late Monday, according to the reports, mortar fire could be heard just beyond Lubumbashi's city limits, in the vicinity of the international airport there.

Lubumbashi is the capital of Shaba Province in southern Zaire, home of the rich copper and cobalt industries that have been traditional economic mainstays. But after years of neglect, the copper industry of Shaba, with 10 percent of the world's reserves, has become a shadow of its former self.

While Lubumbashi teetered toward rebel hands without a fight from government forces, army units still loyal to Zaire's longtime dictator, Mobutu Sese Seko, fought running street battles in Kinshasa with thousands of student protesters who called for the president's ouster.

Near one university campus, units of the Civil Guard and Gendarmerie in armored vehicles chased carloads of students back and forth in a vigorous cat-and-mouse game that lasted for hours Monday morning under a blistering sun.

Elsewhere in the capital, in scenes that recalled the 1989 Tiananmen Square uprising in Beijing, hundreds of students surrounded armored personnel carriers of Mobutu's Presidential Guard, immobilizing the vehicles as they slapped their metal sides with bare hands and harangued the soldiers to help them sweep Mobutu from power.

Many of the soldiers appeared to sympathize with the students, and merely went through the motions of trying to shoo the crowds away.