EAST BERLIN, JUNE 8 -- Hundreds of young pop music fans clashed here this evening for the third straight night with police who barred them from eavesdropping on a concert staged just across the Berlin Wall in the western sector of the divided city.

It was the most serious outbreak of public discontent here in nearly 10 years.

Police arrested several dozen youths, wrestling them into police cars and occasionally beating them with night sticks, along the Unter den Linden boulevard, the main downtown thoroughfare.

The youths, mostly in their late teens or twenties, chanted "Gorbachev! Gorbachev!" in an evident appeal to the authorities of this orthodox-minded communist state to copy some of the Soviet leader's reforms aimed at some loosening up of the means of expression.

The youths tossed small firecrackers and empty wine bottles over lines of green-uniformed police and plainclothesmen who blocked them at several points from coming any closer than 400 yards to the Berlin Wall. Just beyond the Wall, the British rock group Genesis and its star, Phil Collins, were performing on the last night of a three-night rock festival in West Berlin.

"The Wall must go!" and "Down with the pigs!" the youths shouted repeatedly. The clashes tonight followed similar ones, which were on a smaller scale, last night and on Saturday evening.

The disturbances were the most serious ones in East Berlin since a clash following a rock concert in October 1977. Four persons reportedly died in that confrontation, although the deaths were never confirmed by East German authorities.

The incidents came at a particularly embarrassing time for East Germany, which has sought to use this year's 750th anniversary of Berlin as a vehicle for spotlighting the country's successes. East Germany enjoys one of the highest standards of living in Eastern Europe and has shown few signs of political instability.

The clashes are "not the image that East Germany is trying to present, particularly this year," a western diplomat said here.

The street confrontations were likely to sharpen the dilemma facing East German leader Erich Honecker, who has resisted Soviet leader Mikhail Gorbachev's appeals for greater openness and democracy in communist societies, western diplomats said.

East German authorities fear that an easing of restrictions on the media or on public debate could be more disruptive here than in other East European countries, according to the diplomats and other political observers. That is because East Germans -- with ready access to West German television and radio and with family and historical ties to West Germany -- are more likely than other East Europeans to expect western-style freedoms, they said.

About 3,000 young people gathered on Unter den Linden and on side streets this evening. Many stood in small groups around radios and listened to the Genesis concert, which was broadcast live by a West Berlin station.

Most of the arrests were made when crowds surged forward and briefly broke through police lines. Youths struck back with their fists when police hit them with night sticks.

The crowd tonight was somewhat larger than last night, when between 20 and 30 persons were arrested, according to western journalists who were present on both occasions.

Scuffles between youths and police also broke out Saturday night, when a much smaller crowd gathered. No arrests were reported then.

The British pop group Eurythmics performed last night while Saturday night's feature was British rocker David Bowie. All gave open-air concerts on the grounds of the old Reichstag building in West Berlin, which is located just across the wall from the famed Brandenburg Gate at the end of Unter den Linden.

The gate itself, which is readily visible from West Berlin, has become as much a symbol of Berlin's division as the Wall. President Reagan is scheduled to speak in West Berlin directly opposite from the gate on Friday after the Venice economic summit, and U.S. officials have said that he will call for the Berlin Wall to be torn down.

At the rock festival that ended tonight, as in previous musical events at the Wall, the organizers in West Berlin turned some loud speakers toward the Wall so that the music could be audible to East Berliners. It also is normal practice for East Berlin police to block youths from coming close enough to hear, according to western diplomats.

The youths tonight and last night sang the "Internationale" anthem of the world communist movement, in what was clearly meant as a rebuke to the police and a call for them to respect the popular will.

They taunted the police, who generally were impassive except when the crowd pressed forward.

"You should rename this street Stalin Avenue," a young man with a punk hairdo told a policeman.

"Don't you like Genesis? Is this a dictatorship?" another asked.

"This is a democracy," the policeman replied.