As demonstrators took to the streets across the country for a second day on Thursday, police in Portland, Oregon detained several people when anti-Trump protests turned violent, with demonstrators breaking windows of businesses and starting a dumpster fire downtown. Police termed it a riot.

Thousands of Americans took to the streets against Donald Trump's presidential election victory, voicing fears that the real estate mogul's political triumph would deal a blow to civil rights.

Police in Portland, Oregon said that because of criminal and dangerous behaviour, a protest against President-elect Donald Trump is now considered a riot.

In series of Twitter posts, Portland police said they were dealing with increasing vandalism and aggressive behaviour from protesters.

Heated arguments were breaking out at the scene and one video showed an altercation after a woman threw laundry detergent at protesters.

Police said multiple people with bats were reported in the crowd and urged people not wanting to be associated with protesters to leave the area.

About 4,000 protesters were in the streets with chants like "we reject the president-elect."

The Portland protest was among many that occurred across the country.

On the East Coast, there were protests in Washington, Baltimore, Philadelphia and New York, while on the West Coast protesters converged on Los Angeles, San Francisco and Oakland, California. And in Denver, several thousand people rallied in one of the largest of Thursday's events.

Security barricades were erected around two Trump marquee properties ─ the president-elect's newly opened Pennsylvania Avenue hotel in Washington and the high-rise Trump Tower where he lives in Manhattan.

In the nation's capital, about 100 protesters marched from the White House, where Trump had his first transition meeting with President Barack Obama on Thursday, to the Trump International Hotel blocks away.

At least 200 people rallied there after dark, many of them chanting “No hate! No fear! Immigrants are welcome here!” and carrying signs with such slogans as “Impeach Trump” and “Not my president.” “I can't support someone who supports so much bigotry and hatred. It's heart-breaking,” said Joe Daniels, 25, of suburban Alexandria, Virginia.

In Los Angeles, a small band of protesters marched onto a freeway near downtown, briefly blocking traffic until police cleared them away. Minneapolis police reported a similar act of civil disobedience on Interstate 94.

Meanwhile in Colorado, anti-Trump protesters managed to shut down Interstate 25 ─ the major highway─ near downtown Denver briefly Thursday night.

Trump's critics have expressed concern that his often-inflammatory campaign rhetoric about immigrants, Muslims, women and others ─ combined with support he has drawn from the Ku Klux Klan and other white supremacists ─ could spark a wave of intolerance against various minorities.

Trump on Friday tweeted that the protests were 'very unfair'.

"Just had a very open and successful presidential election.Now professional protesters, incited by the media, are protesting. Very unfair!"

Taking a far more conciliatory tone in his acceptance speech early Wednesday than he had at many of his campaign events, Trump had vowed to be a president for all Americans.

Earlier this month, his campaign rejected a Klan newspaper endorsement, saying Trump "denounces hate in any form."

Rudy Giuliani, the former New York City mayor and a high-profile Trump supporter, called the demonstrators "a bunch of spoiled cry-babies," in an interview with Fox News.

'Get it out of their systems'

Republican National Committee spokesman Sean Spicer urged the protesters to give Trump a chance once he is sworn into office in January.

“I hope that people get it out of their systems ... but then they give this man that was just elected very historically and his new vice president an opportunity to govern,” Spicer said in an interview on MSNBC. In San Francisco, more than 1,000 high school students walked out of classes Thursday morning and marched through the city's financial district carrying rainbow flags representing the lesbian, gay, bisexual and transgender communities, Mexican flags and signs decrying Trump.

Several hundred students at Texas State University in San Marcos took to the campus to protest Trump's election.

Civil rights groups and police reported an uptick in attacks on members of minority groups, in some cases carried out by people claiming to support Trump. There were also reports of Trump opponents lashing out violently against people carrying signs indicating support for Trump.

Organisers used social media to plan many of the protests. A Facebook group named “#NotMyPresident,” formed by college and high school students, called for an anti-Trump rally on Inauguration Day, Jan. 20. More anti-Trump demonstrations were planned for the weekend.