Chanting "down with the junta," tens of thousands of Poles defied martial law today by staging an emotional May Day demonstration in support of the suspended Solidarity trade union.

The Solidarity march coincided with an official parade observing May Day that was headed by military and Communist Party leaders. At one point the two processions passed within several blocks of each other, separated only by a line of police supported by water cannon.

Officially conceived as a show of support for the Communist authorities, this year's May Day celebrations in Poland demonstrated that Solidarity is still a force to be reckoned with. The unofficial march, which began almost spontaneously following a mass in St. John's Cathedral in Warsaw's old town, ended as the biggest display of public defiance of martial law yet seen here.

Estimates of the crowd varied between 20,000 and 50,000. Demonstrators dispersed peacefully after singing the national anthem and religious songs such as "God, Return Us a Free Poland."

Parades in support of Solidarity also were reported in other Polish cities including Gdansk, where the trade union movement was born in August 1980.

The marches were the first large-scale, peaceful demonstrations of support for Solidarity since the military crackdown Dec. 13 and could therefore have important psychological consequences for a nation that is gradually overcoming its sense of fear. The leaders of the Warsaw march called for another public demonstration to mark the anniversary of Poland's first democratic constitution Monday.

It was the contrast between the official and unofficial parades that stuck in the minds of many Warsaw residents who watched both go by.

The official procession was well-planned, joyless and silent. It wound its way through almost empty streets guarded by truckloads of police and soldiers. The demonstrators carried carefully painted red banners with slogans like "Peace Yes, War No" and "Our Alliance With the Soviet Union Will Last Forever."

The Solidarity procession was emotional, impulsive and noisy. It gathered size and confidence as it moved through the old town, but it lacked organization and a sense of direction. Wearing Solidarity badges and carrying red and white Polish flags, the crowd demanded freedom for interned Solidarity leader Lech Walesa and the lifting of martial law.

"Free our Lech, imprison Wojciech," the crowd chanted in a reference to the martial-law leader, Gen. Wojciech Jaruzelski.

But the most revealing contrast was in age. The official parade was made up largely of elderly looking Communist Party members and war veterans. The Solidarity march, by contrast, consisted of young students and workers--although a few older people also joined.

Many of the veterans who took part in the official parade remembered the illegal Communist Party marches in Warsaw before World War II. Frequently broken up by the police, these marches gave them a sense of ideological commitment that they still retain.

"Yes, I know the country has gone through many crises under Communist rule," said one veteran in the official procession. "But I joined the party as a youngster, and I feel I have to stick with it."

It was partly the Communist May Day mythology--of defenseless workers being chopped down by the bourgeois police--that made it very difficult for the authorities to suppress the Solidarity march. The crowd understood this and shouted, "Our holiday, our holiday" as they marched.

During the past few days, Solidarity leaflets had been circulating in Warsaw calling on Poles to boycott the official May Day celebrations and attend church services instead. At 10 a.m., when the official procession moved off, the streets of the old town were crammed with Solidarity supporters unable to enter the cathedral.

A group of self-appointed stewards wearing red-and-white arm bands and carrying a large banner reading "Free Solidarity" led the crowd toward the residence of the Roman Catholic primate, Archbishop Jozef Glemp, when the mass ended. Gradually more and more people joined, and the chanting grew louder.

"The police are with us," the demonstrators shouted as they made a turn to avoid a line of grim-faced riot police armed with automatic rifles. Thousands of arms were raised in victory signs.

At the end of the parade, the leaders of the march, who insisted to reporters afterward that they had only just met each other, got up on an impromptu rostrum, a battered old construction van that almost collapsed under their weight. They called for one minute's silence in memory of miners killed by police shortly after martial law was declared.

"I had tears in my eyes, I didn't know what to say. I wanted to cry that all those brave people had come out to join us," said one of the march leaders, a medical student who asked not to be named for fear of police harassment.

A cheer went up when one of the leaders announced that the police had not succeeded in arresting the organizers of a "Radio Solidarity" broadcast interrupted last night after only four minutes. According to official sources, however, a secret transmitter was found during an intensive search by police of apartment buildings in central Warsaw.