I ufSaigon GovernmentSeen TopplhT9 orAs000s Clog RoadsBy ALAN DAWSON SAIGON lt;UP1) — Government authorities urged the300,000 residents of Hue to flee the f ormer imperial capital and it appeared today the city 400 miles northeast of Saigon would fall soon to two North Vietnamese divisions poised for imminent attack.Field reports said residents of the city streamed southward today along the French-built Highway toward Da- Nang, 50 mucs to the south. The new exodus came as tens of thousands of refugees were pouring out of the abandoned Central Highlands toward government-held coastal areas.President Nguyen Van Thieu already had stripped Da Nang of a division of paratoopers and ordered them flown to Saigon to bolster the defenses of theSouth Vietnamese soldiers and civilians, augmented by people living along the jungled dirt route, wound through the central highlands toward coastal havens today.Reports indicated Communist forces were not attacking the convoy. Highly placed government sources told UPI the Viet Cong had guaranteed safe passage as long as the refugees stayed on a designated route.• U.S. Embassy and aid officials today asked volunteer American agency personnel to place themselves on special alert to care for an estimated450,000 to 500,000 refugees fromthe highlands.Government warplanes bombed the abandoned provincial capital of Pleiku today, destroying hundreds of tons of ammunition and two power plants, military sources said.The refugees from Pleiku neared the tiny provincial capital of Cheo Reo today on their way to the relative safety of the coast, the sources said.But the 60-mile long convoy had to stop because demolished bridges blocked the escape route, a dirt road through the jungle.Panicked civilians also fled the resort town of Dalat, 145 miles northeast of Saigon, for fear the government was abandoning the entire central portion of the nation, reports from the scene said.Retreating troops and civilians destroyed and burned mostof Pleiku Sunday and Monday, the military sources said, but they did not have time to level the city and completely destroy all military supplies, so the bombers were sent in.In Saigon, refugees workers said they were toTd to prepare shelter for a half-million people expected to flee the highlands.Government sources said the Communists threatened to launch heavy attacks in the highlands unless Thieu abandoned the provinces. The sources said Thieu agreed despite the opposition of his leading generals.Thieu then ordered South Vietnam's para troop division to the capital from bases in the north,rAi i