Poland's Communist authorities, in an apparent attempt to allay Soviet suspicions of weakness, today briefly detained the country's leading dissident and formally warned him that he was under investigation on suspicion of slandering the state.

Jacek Kuron, who was released after six hours in custody, is the founder-leader of the dissident Workers' Defense Committee (KOR) and a senior adviser to the independent Solidarity trade union federation. Shortly after news of his detention, Solidarity leaders decided to hold an emergency meeting to discuss the case.

The move against Kuron, who was instructed to report twice a week to his local police station, came a day after Kremlin leaders emphasized serious concern over events in Poland. A communique issued after a meeting between Soviet and Polish leaders was interpreted by Western diplomats here as a call by Moscow for tougher action.

Expressing confidence "that Polish Communists had the capability and strength to change the course of events," the communique said the Warsaw Pact allies would provide all needed support for the Polish leaders "in their tense work to improve radically the situation in the country."

The communique issued after the Moscow summit appeared deliberately ambiguous on the vital question of what specific steps the Polish authorities would take to reassert their control. Political analysts here say this ambiguity may have reflected an apparent difference of approach between Warsaw and Moscow.

While Soviet leaders clearly favor action to limit Solidarity's influence, the Polish government is seeking to pursue policies that will restore public trust and confidence. In the short term, at least, this means cooperating with the new unions rather than challenging them in trials of strength.

Gen. Wojciech Jaruzelski, the new premier, has attempted to take a more consistently conciliatory line towards Solidarity than his predecessor, Jozef Pinkowski, whose alternating tough-soft approach resulted only in wave after wave of strikes.

It is difficult to see how the Polish authorities can roll back Solidarity's gains without provoking widespread unrest.

Meanwhile, several labor disputes loomed that could strengthen a fragile three-week truce between Solidarity and Jaruzelski's new government. The Solidarity branch in Lodz, a major textile city in central Poland, has called a strike alert in protest against the dismissal of five workers from a military hospital.

In Siedlice, east of Warsaw, token protests were called for Friday against the continued detention of seven right-wing dissidents. Workers plan a poster and leaflet campaign.

The threats of new disruption, along with evidence of Soviet impatience, illustrate the difficulty of the balancing act now required of the Polish Communist Party leader, Stanislaw Kania. Any action he takes against alleged "anti-socialist forces" must be tough enough to satisfy Moscow but mild enough to avoid a backlash from Solidarity.

Solidarity leaders gave tentative support to a call for a 90-day moratorium on strikes by Jaruzelski when he became premier last month. But they have made clear they will strongly resist moves by the government against their members or advisers.

Kuron, who has been detained dozens of times in the last decade, is one of a large number of KOR members or sympathizers employers by Solidarity in an advisory capacity. He has helped Solidarity leader Lech Walesa on organizational questions and political tactics.

Last month, the chief state prosecutor announced that legal proceedings for alleged "anti-state activity" had been started against KOR. It was established in 1976 with the original aim of defending workers harassed by police. The organization played an important role during last summer's strikes by acting as an information center, collating and distributing reports of labor unrest.

Despite the new pressure from Moscow, the Polish authorities have gone ahead with the publication of a liberalized draft labor law. The draft formally recognizes the right to strike but allows the legislature to ban strikes for a period of up to two months in any one year in the event of a national emergency.

It also bypasses the controversial issue of whether private farmers should be allowed to establish independent unions.