ZAGREB, Croatia, May 3— Rebel Serbs blasted the Croatian capital today with the second rocket attack in two days, killing one policeman and wounding 43 civilians, before agreeing to a tenuous cease-fire.

City authorities said five rockets hit central Zagreb, including one that went through the roof of the Academy of Dramatic Arts, wounding Russian, British and Ukrainian ballet dancers rehearsing for a performance.

One policeman was killed trying to defuse a cluster bomb carried by another rocket that fell near the city's main children's hospital.

The attack was similar to one on Tuesday that killed 5 people and wounded 134. Once again, the center of this city of one million people fell silent after several deafening explosions. In the elegant Mazuranic Square, where the National Theater and the Academy stand, military policemen dug for rocket fragments among freshly-planted tulips.

Tram service in the city stopped, and people rushed to their homes as the rockets began falling.

The attack coincided with talks between Yasushi Akashi, the top United Nations official in the former Yugoslavia and separatist Croatian Serb leaders in their stronghold of Knin. Later Mr. Akashi announced a cease-fire, saying, "I have the words of honor from both sides to implement the agreement."

Four years of fighting, however, have littered the Balkans with broken cease-fires bound both by words of honor and written accords.

Most recently, ignoring a "permanent cease-fire" signed last year, the Croatian Army swept through a Serbian enclave in western Slavonia, about 75 miles southeast of the Croatian capital, in an offensive that prompted the Serbian rocket attacks.

The four-point cease-fire negotiated by Mr. Akashi states, "There will be a total cessation of hostilities in western Slavonia and elsewhere." By "elsewhere," the accord apparently refers to the rebel Croatian Serb positions about 25 miles south of Zagreb from which the rockets were fired today and Tuesday.

The agreement, which was due to go into effect this afternoon, adds that civilians and soldiers who wish to leave western Slavonia may do so under United Nations surveillance. Soldiers, all of whom would presumably be Serbs, will be allowed to take sidearms only, with all other weapons to be handed over to the United Nations.

The most critical point of contention, however, is not addressed by the agreement. That is whether any attempt will be made by the United Nations to oblige the Croatian forces to leave the western Slavonian pocket and return to the lines they occupied before the current offensive. Croatian Serbs insist this is essential; the Croatian Government vehemently rejects the idea.

"We cannot accept a demand for withdrawal for the simple reason that the territory liberated was always Croatian territory taken away from us by force in 1991," said Natasha Rajakovic, a spokeswoman for President Franjo Tudjman.

But Ilija Prijic, a senior Croatian Serb official, insisted that the Serbs understood the agreement reached today as calling for Croatian forces to withdraw within the next 24 hours from the 200 square miles of territory they have captured in western Slavonia.

The disagreement did not bode well for an accord that was clearly put together in haste by Mr. Akashi in an attempt to avert renewed full-scale fighting in Croatia.

Fighting erupted here in 1991 after Croatia broke away from Yugoslavia and the region's Serbs, about 13 percent of the population at the time, resisted the secession. About 10,000 people were killed in the conflict, which ended in a tenuous truce that left the Croatian Serbs holding about 30 percent of the country.

The United Nations Security Council and several Western governments have suggested that Croatia must now return to the previous cease-fire lines in western Slavonia.

But these appeals appear to ignore the enormous emotional lift the Croatian military victory has brought here and the fact that four years of laborious international negotiations on Yugoslavia's breakup have never shifted a front line or a border by an inch. These front lines and borders have only been altered by force.

Until recently, the Serbs, who inherited most of the ordnance and officers of the former Yugoslav Army, have shown a military superiority in both Croatia and Bosnia and thus dictated the borders.

But the crushing Croatian victory in western Slavonia -- albeit against the most vulnerable piece of Serbian-held land in Croatia -- suggests that superiority may be fading. Moreover, the Bosnian Serb forces, stretched in Bosnia by a long front line and by an improving Muslim-led Bosnian Army, did not come to the aid of their Serbian allies in Croatia.

The rocket attacks on Zagreb -- random and murderous -- have suggested that Serbian forces are bereft of a strategy, and there has been no coherent military response to the Croatian offensive.

Among the dancers hurt today was Mark Bolden of Britain, who had surgery to remove shrapnel from his abdomen, a British Embassy spokesman said.

Today in Novska, on the western edge of the Slavonia pocket captured by the Croatian Army in a in a 36-hour pincer attack on Monday and Tuesday, Croatian refugees who fled the enclave in the 1991 war spoke enthusiastically of the victory and their plans to go home.

"Over the past four years we have been shifted to five different places," said Maria Karavla, a refugee from the village of Rozdanik. "My husband went back to our village for the first time today. Our house has been burned down but we plan to start again and rebuild it."

Several refugees said President Tudjman had waited too long in using force. They urged him to move now to retake other Croatian territory held by Serbs.

For all the Croatian refugees about to return home in western Slavonia, there are now at least 5,000 Serbian refugees trudging from the area into Serbian-held parts of northern Bosnia. As long as these movements of population continue, more fighting seems inevitable.

Photo: A group of Croatian boys and girls took shelter yesterday in the basement of Zagreb's main children's hospital during a Serbian rocket attack. It was the second day that Zagreb was shelled by rebel forces. A policeman was killed trying to disarm a cluster bomb that landed near the hospital. (Associated Press) Map of Bosnia and Herzegovina showing locations of Zagreb and areas of control.