‘Radical’ Bulgaria Change Put Next : East Bloc: Reformers hail the new chief. Stress is on economic restructuring and the need to further rid the leadership of hard-core elements.

Leaders of the small Bulgarian reform movement said Saturday that they will push for “further radical changes” in the wake of the dramatic transfer of leadership that ended the 35-year rule of Todor Zhivkov, the longest-serving leader in Eastern Europe.

Dr. Petar Berom, Sofia University professor and one of the leaders of the reform-oriented organization Eco-Glasnost, said several groups plan to join in a demonstration next week in support of new Bulgarian leader Petar Mladenov. Mladenov, 53, foreign minister since 1971, was unexpectedly named Friday to replace Zhivkov, 78, as general secretary of the Bulgarian Communist Party.

Zhivkov’s forced resignation here in Bulgaria, a Balkan country of 9 million sometimes called the “16th republic of the Soviet Union” because of its steadfast loyalty to Moscow, leaves only Romania and Czechoslovakia in the East Bloc without a significant change of leadership during the recent period of Soviet-inspired reforms.

“We want to make sure that the reforms will not stop with the change at the top,” Berom said. “At the same time, we want to demonstrate support for our new leader and help him get rid of the hard-core elements in the government.”

Joining Eco-Glasnost in the demonstrations planned for next Saturday, Berom said, will be members of several other Bulgarian reform groups, including the Perestroika-Glasnost Support Club, the Independent Committee for Human Rights, the Committee 273 (a group concerned with political prisoners) and members of the Helsinki Watch organization.

Despite the change in leadership that ended 3 1/2 decades of rule for Zhivkov, a World War II resistance leader who in recent years was condemned by international rights organizations for his policies affecting the minority Turkish-speaking population here, Western diplomats said they were surprised by the muted public reaction Saturday to the dramatic power shift.

“I guess they’re just not very demonstrative people,” said one.

In fact, in contrast to the tumultuous events that have taken place in much of Eastern Europe recently, Bulgaria’s reform movement is small potatoes. The country’s first anti-government street demonstration, in which an estimated 9,000 people gathered near the capital’s Alexander Nevsky Cathedral on Nov. 3, lasted only 15 minutes before participants voluntarily disbanded.

Bulgarian newspapers Saturday stressed the economic aspects of reform under the new leadership, not the more radical political elements of the Soviet-model experiment.

Bulgarian TV news simply repeated the contents of speeches made by leaders on Friday.

In the eyes of some of the opposition leaders, Mladenov--who suffers from poor health, including ailing kidneys that require dialysis treatment--does not represent the vanguard of the reform-minded leaders, even within the Communist Party.

Andrei Loukanov, minister of foreign trade, and Ivan Panev, chief of the party in Sofia, are more strongly identified with the reform movement here.

In his speech on Friday during the meeting of the Central Committee of the Bulgarian Communist Party that named him to succeed Zhivkov, Mladenov made a point of stressing the continuity of the Communist-socialist system here.

Speaking of a planned economic “restructuring” initiated under Zhivkov, Mladenov said, “This can be realized only and exclusively in the socialist system. . . .”