SOFIA, BULGARIA, JULY 6 -- President Petar Mladenov submitted his resignation today, bowing to mounting pressure from student demonstrators and the political opposition.

Mladenov's letter of resignation was read on television at 8 p.m., meeting a deadline set by the students in an ultimatum issued earlier in the day. The students chanted "Victory! Victory!" when they heard the news, the state BTA news agency reported.

They have been conducting a sit-in for nearly a month to demand democratic change after Mladenov's Socialist Party -- a reform-minded, renamed incarnation of the Communist Party -- won last month's parliamentary elections, the first free balloting in Bulgaria in 58 years.

The students demanded Mladenov's resignation after he suggested that tanks be used against anti-government demonstrators last December.

Mladenov, who was instrumental in ousting Bulgaria's longtime Communist strongman, Todor Zhivkov, said he was resigning because he did not want "to become the cause for an escalation of tensions in the country," BTA reported. Students, teachers and parents from across the country joined in the student demonstration in Sofia, and the growing split in public opinion led to fears of violence.

Premier Andrei Lukanov appeared on television shortly after Mladenov's resignation was announced and appealed for order so the national legislature could open Tuesday in a calm atmosphere.

About 1,000 student demonstrators had gathered in front of the presidential building since Wednesday to demand Mladenov's resignation. They had called for a 30-minute warning strike if Mladenov did not step down tonight and a general strike on Monday if he had not resigned by then.

The Union of Democratic Forces, an umbrella organization of 16 opposition parties, issued a statement also urging Mladenov to resign. Trade unions, other opposition groups and the mainstream Agrarian Party also backed the students' demands, BTA reported.

Mladenov, in a nationwide television address on Wednesday, implicitly admitted having called for the use of tanks during the December demonstrations. He commented after experts said a videotape in which he said, "Let the tanks come," was authentic. He had earlier said the video had been doctored.

The film was made Dec. 14, when about 50,000 angry demonstrators converged on the legislature to demand that the Communist Party give up its monopoly on power. The protesters later dispersed peacefully and tanks were not called in.

Mladenov came to prominence in November when he and other reform-minded Communists ousted hard-liner Zhivkov after 34 years.

Mladenov replaced Zhivkov as Communist Party leader and president. He began taking cautious steps toward reform and opened a dialogue with opposition groups.