Vice President Mondale arrived in this ancient capital to applause from tens of thousands of people lining both sides of his motorcade route, apparently the first such mass welcome ever received by a U.S. official visiting Communist China.

The huge turnout, of the sort Peking authorities usually have discouraged in recent years, seemed to have been encouraged by local leaders. But the broad grins of men, women and children and shouts of "friendship" and "old friends" along the 10-mile route appeared genuine.

One thousand years ago this was the largest and most modern city in the world, but history has moved away from Xi'an, and its 2.7 million people seemed flattered that Mondale had decided to touch down here for 23 hours. Their enthusiasm and curiosity also may have been stimulated by Mondale's nationwide speech broadcast Monday from Peking and the government's general happiness with progress in Sino-American relations.

Mondale told a welcoming banquet soon after his motorcade reached a government guest house, "A few minutes ago, I had one of the most moving experiences of my political life. The welcome of the people of Xi'an extended to me was truly overwhelming."

A local Communist Party official and American aides who had prepared for the visit said they made no attempt to organize a crowd.

"People just read in the newspaper or heard on the radio that he was coming, and the road is the only way to the guest house," the Chinese official said.

One party official estimated the crowd at 200,000, but U.S. officials said it was too difficult to make an accurate count.

Mondale left Peking earlier today after a four-day visit, which included his unprecedented speech, signing of a new cultural and a hydroelectric agreement and the announcement of a visit to the United States next year by Communist Party Chairman Hua Guofeng, Vice Premier Deng Xiaoping, who conducted most of the talks with Mondale, rode with him to the airport, an unusual courtesy to a visitor who is not a head of state.

Administration aides seemed surprised by the motorcade reception here. Many U.S. television viewers who watched President Nixon's first visit to Peking in 1972 were struck then by the indifference of Chinese watching the motorcade. American China experts are accustomed to only curious stares from passerby as they ride motorcades in from airports at Peking, Shanghai or Canton.

"We stopped organizing parades a long time ago in Peking," said one Chinese journalist, recalling that in the early years the party did turn out some large welcoming crowds. But people in Xi'an, capital of north central (Shenshi) Province, do not see long motorcades very often.

Xi'an ("Western Peace"), known in ancient times as Changan ("Eternal Peace"), was the capital of China off and on for more than a millenium. It reached its peak 1,000 years ago when it had more than a million people and ruled an empire of enormous scope and riches.

In China, Xi'an "historically is the equivalent of Peking," said Harvard History Professor Emeritus John K. Fairbank, who is accompanying Mondale. In recent years, Xi'an has enjoyed more modest influence as the provincial capital and as a growing industrial center.

The vice president plans to visit an oil exploration instruments factory as well as one of the blossoming rural free markets. Xi'an was included in the trip as a good sample of China's interior and as a center of some of the most spectacular archeological finds in the world, a subject that interested Mondale and his artist wife, Joan.