Demonstrators took to the streets of Miami, New York, Portland and other US cities for a third straight night of nationwide protests to oppose Donald Trump's election as president.

People gathered in New York's Washington Square neighborhood in lower Manhattan on Friday, some carrying enormous red balloons and placards with hearts and the words "peace and love".

In Portland, Oregon, a man was injured in a shooting which city police believe resulted from a confrontation with someone at that protest.

According to witnesses cited by the New York Times, the altercation began when a car full of people grew angry about being stalled in traffic and one protester was shot in the leg.

Prior to the shooting Portland police reported on Twitter that projectiles were "being thrown at officers" for a second night, while local media said that police in riot gear used pepper spray and flash-bang devices to scatter the crowd.

In New York, the local ABC affiliate said an estimated 4,000 people gathered at Washington Square, while others descended on Union Square and marched through Midtown. Still more converged outside Trump Tower.

Some at Washington Square held signs reading "Your Wall Can't Stand in Our Way" -- a reference to the anti-immigration barrier the billionaire real estate baron has promised to build on the US border with Mexico.

The New York Police Department reported that 11 people had been arrested as of late Friday, the New York Times said.

Demonstrators said they wanted to show solidarity with those they felt may be targeted by Trump's policies once he takes office in January, including Mexicans and Muslims.

"We're here to support the people that Trump has insulted, to show our children that we all have a voice, and to stand up for people's rights," Kim Bayer, 41, told AFP.

Organizers plan another large demonstration in the same Washington Square location on Saturday, and more protests are expected across a number of cities over the weekend.

Meanwhile, about a thousand protesters took part in a demonstration in Miami, hoisting signs against racism as they walked down Biscayne Boulevard opposing the deportation of undocumented immigrants.

At a small protest in California, about two dozen women wearing shorts and sports bras locked arms and staged a brief flash mob-style protest in the middle of a freeway south of Los Angeles on Friday, causing traffic to come to a standstill.

Several of the women carried placards that read "Unity" as they marched down the four-lane freeway in Costa Mesa.

Atlanta police estimated that the city's largest anti-Trump protest yet drew more than 1,000 people on Friday, according to local broadcaster WSB. Outside the Georgia Capitol, protesters reportedly burned a US flag.

And in Philadelphia, some 250 people gathered for a rally against the president-elect, ABC affiliate WPVI reported.

Further north in Boston more than a thousand people converged in the afternoon for a "Love Rally" against Trump's divisive rhetoric, local media said.

Demonstrations also occurred in Detroit, Dallas, Memphis, Orlando, Raleigh and a number of other cities.

Since Trump's shock victory on Tuesday over Democrat Hillary Clinton, who had been widely tipped to win, thousands have protested in cities across the United States.

READ: Trump election win sparks protests across US

While most of the demonstrations have been peaceful, officials said some erupted in violence that caused property damage.

There has also been an increase in violence by both Clinton and Trump supporters against partisans on the other side, law enforcement officials said.