Leonid Brezhnev, the Soviet President, has sent a warm message of congratulations to the new Polish Communist Party leader, coupled with a call to defend Poland against ''counterrevolution.''
The message expressed ''confidence'' that the new party chief, Premier and Defense Minister Wojciech Jaruzelski, would prove equal to the task.
Diplomats here interpreted the congratulatory note, which was carried by the official Soviet news agency, as a sign the Kremlin was generally pleased with the latest power change in Warsaw. Mr. Jaruzelski replaced Stanislaw Kania as Communist Party leader Oct. 18.
But the diplomats also stressed that the Kremlin's ultimate judgment on the Polish power change would depend on the extent to which Mr. Jaruzelski - whom the Soviet message called a ''prominent party and state leader of . . . great prestige'' - managed to shore up the Polish party and curb militant Polish reformists.
''Jaruzelski is probably, in theory, more capable of delivering (these things) than was Kania, who had lost credibility with virtually all Polish forces,'' one diplomat said. ''But it's an extremely delicate task, and it's just too early to tell what the results of the power change will be.''