In December 1981, Jaruzelski suddenly declared martial law, ordering the army and special police units to seize control of the country, apprehend Solidarity's leaders, and prevent all further union activity. In effect, Jaruzelski executed a carefully planned and efficient military coup on behalf of the beleaguered and paralyzed the PZPR. The motives of this act remain unclear. The general later claimed that he acted to head off the greater evil of an imminent Soviet invasion; detractors dismissed this explanation as a pretext for an ironfisted attempt to salvage party rule. In any case, the junta suppressed resistance with a determination that cost the lives of several protesters, and by the new year the stunned nation was again under the firm grip of a conventional communist regime.

Under martial law, Jaruzelski's regime applied draconian restrictions on civil liberties, closed the universities, and imprisoned thousands of Solidarity activists, including Walesa. During the succeeding months, the government undid much of Solidarity's work and finally dissolved the union itself. Official pressure overcame repeated attempts by Solidarity sympathizers to force the nullification of the December coup. By the end of 1982, the junta felt sufficiently secure to free Walesa, whom it now characterized as the "former leader of a former union." After gradually easing the most onerous features of the state of emergency, Warsaw lifted martial law in July 1983, but Jaruzelski and his generals continued to control the most critical party and government posts.