Some 20,000 Italian Communists held a mass meeting at Rome ultra-modern Palace of Sport yesterday (Sunday) to celebrate the 50th anniversary of the Italian Communist Party. The Italian Party is the largest in the west and the second biggest political group in the country's parliament.

The Party in fact reached its anniversary last week, but last night's rally was the main celebration, and it was attended by official delegates from the Soviet Union. Among the platform party were Signor Luigi Longo, the Party leaders, and Signor Enrico Berlinguer, his deputy.

In his speech, Sr. Longo re-affirmed that Italian Communists were following their own road "to a socialism that responds to the national traditions, to the requirements of our country and of our country alone." The Party was no seeking special privileges, he said; it was already a party of government even thought it was not in the ruling coalition.

The Party, which claims a million and a half members and received more than 8 1/2-million votes in the 1968 elections, has 171 deputies in the Chamber of Deputies (lower house) -- second only to the Christian Democrats.