Thousands of people rallied today in Burma's two largest cities to demand an end to authoritarian rule, diplomats said, and the government prepared to choose a replacement for the nation's ousted leader. An Asian diplomat in the capital of Rangoon cited reports that 100,000 people, including students and Buddhist monks, marched along the streets of Mandalay, the nation's second-largest city, 350 miles north of Rangoon. According to 1984 figures, Mandalay is a city of 600,000 people. A Western diplomat in Rangoon said at least 1,000 protesters were present at the start of a demonstration in front of Rangoon General Hospital. It was the third straight day of demonstrations in Burma's capital. Japan's Kyodo News Service, reporting from Rangoon, later put the number of Rangoon protesters at 15,000. It also said thousands rallied in Mandalay. Security forces made no attempt to interfere with the peaceful protests in both cities, said the diplomats, speaking on condition of anonymity. Kyodo said soldiers armed with bazookas were seen in parts of Rangoon, along with tanks and armored personnel carriers. Meanwhile, accusations mounted that Rangoon's security forces shot unarmed civilians last week during massive anti-government riots. Estimates of those killed range from 95 to more than 3,000. Security forces allegedly shot doctors and nurses outside the hospital during the rioting, which ended the 17-day rule of hard-line President Sein Lwin. Sein Lwin resigned last Friday as state president and chairman of the Burma Socialist Program Party after five days of massive protests. He had replaced Ne Win, who resigned last month after 26 years of authoritarian rule. Diplomats, speaking on condition of anonymity, cited reports that the party's central committee of the Burma Socialist Program Party has been meeting daily since Sein Lwin's ouster to prepare for an emergency party session Friday, presumably to choose a new chairman. The legislature also is to hold an emergency session the same day, presumably to name a new president. The state-run Radio Rangoon said 52 people arrested in last week's rioting were freed today, including 43 students. According to official reports, 155 people had been released in Rangoon since Tuesday. Several prominent figures arrested July 29-30 still are being held. They include the country's leading dissident, Aung Gyi. Sein Win, the Burmese correspondent of The Associated Press, also remains in custody. U.S. State Department officials and a number of Rangoon diplomats say they can at best estimate the number of dead nationwide as ``in the hundreds'' given the limited movement of diplomats last week in Rangoon and elsewhere. Dozens of tourists and some journalists witnessed the mass demonstrations last week in Rangoon, but they returned with vague accounts of killings. Only one photograph of a corpse was seen in Bangkok. One senior Western diplomat told newsmen that a grisly ``battle of the bodies'' accounted in part for the lack of information about numbers killed. ``When someone is killed, (the protesters) try to get it so they can display the bodies and government tries to get it to cremate it,'' he said. After demonstrations in March, diplomats learned that the bodies of a number of young protesters gunned down by police had been taken swiftly to a cremation ground and burned. Bertil Lintner, a Swedish journalist and Burma scholar, says 3,000 persons were killed in Rangoon alone, including 600 who died at the Rangoon General Hospital. He was not in Rangoon last week, but cited witnesses to the demonstrations he later interviewed by telephone from Bangkok.