To the accompaniment of deafending bus horns and gleeful victory signs, Polish transport workers today ended a 50-hour demonstration in central Warsaw called to protest mounting food shortages.

The lifting of the blockade of one of the capital's busiest traffic intersections coincided with the end of a two-hour warning strike that affected most factories in the surrounding region. Further strikes and demonstrations are planned in other parts of the country unless there is progress in the next stage of talks, due to be held Thursday, between the communist authorities and independent trade union federation Solidarity.

The street demonstrations have injected a potentially dangerous new element into Poland's labor crisis, which blew up a year ago with massive workers' strikes along the Baltic Coast. Until the last two weeks, Polish strikers had remained inside their factories and thus avoided any direct conflict with the police.

So far, however, the demonstrations have passed peacefully and both sides have tried to prevent passions from getting out of hand. At times, this week's protests in Warsaw even took on the character of an exuberant street festival, with two of Poland's best known cabaret artists entertaining the large crowds that gathered in the city center.

As the blockade broke up and a long line of 150 buses, streetcars and trucks finally moved off, passers-by stopped to cheer and throw flowers. Office workers and shop clerks in the high buildings overlooking the city's central traffic circle on Marszalkowska Street also shouted encouragement at the demonstrators.

Originally, Solidarity officials had planned the traffic protest to last just a couple of hours on Monday morning. But it turned into a partial blockade of the city center when the police refused to allow the vehicles, which were decorated with slogans and red and white national flags, to turn down a street leading past the offices of the Communist Party and government.

In the end, after two days and nights of stalemate, the autorities won their point. But Solidarity activists said the demonstration had served to attract national and international attention to the extent of the food crisis here.

Watching the buses and streetcars move off, crammed with jubilant Solidarity supporters waving to the crowds, a Soviet journalist based in Warsaw remarked: "After 12 months, the authorities have finally scored a great victory -- they've prevented a line of vechicles from making a left turn."

Yet, while the confrontation may have begun over the "left turn" issue, it quickly assumed more importance. 'it turned into a show of strength by Solidarity at a time when negotiations with the government over the scope and nature of economic reform had reached a stalemate.

Solidarity is insisting that planned price increases (of up to 300 percent in some cases) can only be considered in the context of the structural economic reform based on the independence of individual factories and the introduction of a market-type economy. The union has also reiterated its old demands for greater access to the news media and greater social control over food distribution.

The government argument is that economic reforms need careful preparation and strikes and other forms of protest can only make the shortages worse. Official commentators also have accused Solidarity leaders of playing with fire by attempting to engineer conflicts with the authorities.

The Army paper, Zolnierz Wolnosci, commented today: "Who can guarantee that no one will throw a spark on this gunpowder of surging emotions? There are an abundance of myopic fools, blind fanatics, and simply enemies. The consequences of their actions could be incalculable."

Sympathies among ordinary passers-by on Marszalkowska Street seemed firmly with Solidarity today despite the dislocation caused by the protest. Few people expected any immediate improvement in food supplies or Poland's desperate economic situation, but said the demonstration was necessary to relieve tensions.

One elderly housewife, who said she had gotten up at 5 a.m. to search for bread and meat, expressed pride in the demonstration.

"The people have shown their power very clearly," she said as trucks and buses rumbled past emblazoned with slogans like, "We demand restoration of full rations," We want bread" and "Warsaw waterworks supports the protest."

Police watched impassively as the procession passed, but some accepted red carnations from Solidarity officials, which they clutched next to their nightsticks. In an attempt to prevent incidents, Solidiarity had organized its own cordon of steelworkers to keep the crowds back and separate them from the police.