UN Secretary-General Ban Ki-moon on Friday welcomed a call from Donald Trump, the U.S. president-elect, for unity after what has been described as a bitter and divisive presidential campaign.

In a phone call on Friday afternoon when anti-Trump protests were underway across the United States, Ban congratulated Trump again on his election as the forty-fifth President of the United States of America, UN spokesman Stephane Dujarric told reporters here.

"The secretary-general welcomed President-elect Trump's calls for unity following a hard-fought election," the spokesman said.

Since Trump's election victory on Wednesday, people took to the streets in such U.S. cities as Boston, Chicago, Detroit, Miami, New York, Philadelphia and San Francisco to protest the billionaire's election. The protests continued into Friday night.

"The secretary-general expressed confidence that the United States and the United Nations would maintain their traditional strong ties of cooperation to advance peace and security, sustainable development and human rights throughout the world," the spokesman said.

"They agreed to stay in continued contact," he added.

Ban is to leave his office on Dec. 31 at the end of his two five-year terms while Trump is to be sworn in as the U.S. president on Jan. 20.