Poland's national assembly moved today to strengthen the country's police apparatus and write into the constitution broad powers for declaring a state of emergency. The steps anticipated possible government motions next week abolishing the 1981 martial-law decree that crushed the Solidarity independent labor union movement.

Meanwhile, the leadership of the Solidarity underground belittled the expected lifting of martial law as "only a gesture without any substantial significance," noting that the government has no intention of canceling legislation enacted "under the cover" of military rule that "restructured society and granted broad police powers."

"A merely formal lifting of martial law and amnesty will in no way induce us to halt our activities," the clandestine, five-member Solidarity Coordinating Council said in a statement reaching western correspondents here today. The council's members have been in hiding since military rule began 19 months ago.

Polish authorities have hinted strongly that the assembly, or Sejm, will be asked to affirm the abolition of martial law during a scheduled session July 20 and 21. Fueling speculation about a major announcement then, it was reported officially today that the Polish leader, Gen. Wojciech Jaruzelski, would address the Sejm next Wednesday.

There were indications, however, that the Communist leadership plans to carry over some martial law-style regulations for months to come. Among the legislation introduced today was a constitutional amendment to give the government for the first time power to impose a state of emergency. Another proposed amendment fulfills a Communist Party pledge to guarantee the right to own private farms.

Another bill was passed today strengthening the security functions of the Internal Affairs Ministry, which is headed by Gen. Czeslaw Kiszczak, a staunch Jaruzelski ally.

The new legislative package contains restrictions considered by authorities as still necessary to grapple with Poland's disastrous economy and entrenched underground opposition.

These measures are believed to represent a compromise between conservatives in the Warsaw leadership who wanted to keep martial law in its currently suspended status longer and sought permanent, tough changes in existing laws, and those who favored an earlier lifting of martial law.

Sejm deputies have yet to receive copies of these proposed new regulations. The senior leadership also has not yet forwarded any bill covering an amnesty for political prisoners.

The delay may reflect backstage quarreling in the leadership or an attempt to gauge western reaction before proceeding. The Reagan administration has stated its willingness to roll back some economic sanctions against Poland if a significant number of political prisoners are released when martial law formally is ended.