Street fighting flared in four major Polish cities yesterday as riot police dispersed demonstrators seeking to protest nine months of martial law, according to official accounts published here today.

The official Polish news agency PAP reported that a column of police trucks was ambushed late last night in the southwestern city of Wroclaw. It said stones and bricks were hurled down at the convoy from buildings and four policemen were seriously injured when their van crashed.

A march in support of the suspended Solidarity trade union also took place in the southern industrial town of Nowa Huta. The government's spokesman, Jerzy Urban, told reporters today that between 200 and 300 workers had been joined by groups of youths. A hard core of demonstrators was later dispersed by riot police using water cannons and tear gas.

The latest incidents were on a much smaller scale than the nationwide protests underground Solidarity leaders organized to mark the second anniversary of the Gdansk agreement that recognized independent trade unions on Aug. 31. But they are nonetheless taken as evidence of a depth of feeling against the martial-law regime.

The underground Solidarity leadership has formally appealed to union members not to take part in uncoordinated street demonstrations, which police can break up with relative ease. Its strategy is to wait and see whether the government is prepared be conciliatory to avert long-term instability.

So far, the military authorities have assumed an unbending stance in public. This has been combined, however, with promises to increase workers' living standards and an official denial that ground is being prepared for the formal outlawing of Solidarity.

In what was an embarrassing coincidence for the authorities, yesterday's clashes coincided with the visit to Moscow of the new Polish foreign minister, Stefan Olszowski. One of the purposes of his visit, it was assumed here, was to reassure the Kremlin that the Polish government has the situation under control.

In Moscow, a joint communique issued after Olszowski's departure following the two days of talks made no mention of Poland's latest difficulties, Washington Post Moscow correspondent Dusko Doder reported. The communique did quote Olszowski as asserting that ties to Moscow remain "the keystone of Polish foreign policy," acquiring "still greater significance" in the present complex international situation.

Urban, at his press conference, took pains to downplay the significance of the disturbances yesterday, which he attributed to groups of "excitable youths." He revealed, however, that attempts to organize demonstrations also took place in the central textile city of Lodz and the northwestern seaport of Szczecin. The 13th of every month has become a trigger for unrest because it was last Dec. 13 that Gen. Wojciech Jaruzelski declared martial law.

Giving details of the disturbances in Wroclaw, PAP said that 15 people were arrested and charged with assault on the police after "a bandits' attack" on a column of police vehicles. No independent accounts of the incident were immediately available. PAP said the attack had taken place at 10:30 p.m. -- well after the start of the nightly 8 p.m. curfew that was imposed on Wroclaw following big demonstrations there Aug. 31.

PAP said that earlier in the evening, 59 people were detained when they attempted to demonstrate after a religious mass. A center of heavy industry with a population of more than 600,000, Wroclaw has long been a Solidarity stronghold.

A recent clandestine Solidarity bulletin said that several officially unreported attacks on police vehicles have taken place there since Aug. 31.

A similar mood of defiance appears to have gripped workers at the giant Nowa Huta steelworks. When built in the early '50s, it was conceived as a bastion of Communist support to offset the influence of nearby Krakow, Poland's religious and cultural capital.

Three or four marches have been reported from Nowa Huta during the past month. As in the past, yesterday's demonstration began after the first shift streamed out at 2 p.m. Urban said "several dozen" demonstrators were arrested after refusing orders to disperse.