President-elect Donald Trump in Washington on Thursday. Photo by AP?

It was still dark in Washington, DC, at 6:20 am on Friday as Sage Sopko and his friend Jonathan Kingery walked out of the Metro Center rapid transit station wearing red caps emblazoned with "Make America Great Again".

That was the campaign slogan of New York real estate billionaire Donald Trump, who was inaugurated around noon on Friday as the 45th president of the United States.

Sopko and Kingery, both of whom are students at Illinois State University some 1,200 kilometers away, had skipped school for the inauguration.

"We are Trump fans," Sopko said. "If we skip school, this would be the best time to do it."

Kyle Houk, from Cincinnati, Ohio, also emerged from the Metro station. "I am very excited today," said Houk, who previously attended three Trump rallies in Ohio.

The Metro was unusually crowded early on Friday morning. So were the streets, where National Guard soldiers stood at checkpoints and huge concrete traffic barriers had been installed.

With Republicans now in control of the White House and both houses of Congress, around 70 Democratic lawmakers announced this week that they would not attend the inauguration ceremony.

Many demonstrations for or against Trump had been scheduled for Friday and over the weekend. The largest protest, Women's March on Washington, is set for Saturday morning and was expected to draw around 200,000 people.

Li Haidong, a professor of US studies at China Foreign Affairs University, said the new US president will face challenges.

"The first year after Trump takes office, he might push the wheel of history back … because he has anti-globalization policies in terms of economy and security," Li said.

But Ning Jizhe, chief of China's National Bureau of Statistics, said he expected stronger China-US economic cooperation after Trump is sworn in.

Ning told a news conference in Beijing on Friday that he believed Trump would consider issues from the perspective of developing mutually beneficial ties and would advance the long-lasting cooperation between the two major economies. "I am hoping for that," he said.

Jin Canrong, a professor of international relations at Renmin University of China, said, "Overall, there is no need for China to be over-worried about Trump. Instead, Beijing should keep calm."

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