Opponents of Boston white nationalist rally slammed by Trump

US President Donald Trump described anti-Nazi demonstrators in Boston protesting white nationalism as "anti-police agitators" on Sunday (NZ time).

In a tweet he wrote, "Looks like many anti-police agitators in Boston. Police are looking tough and smart! Thank you."

Around 15,000 counter-protesters marched through the city to historic Boston Common on Sunday, outnumbering a right-wing group, which had its rally cut short.

The event was billed as a 'Free Speech Rally', but appeared to end before it began after they were escorted off the grounds by police, less than an hour after the event was underway. 

Organisers of the event, The Boston Free Speech Coalition, said on Facebook it was not affiliated with white nationalism, racism or the Charlottesville rally organisers in any way.

Footage of the event showed police vans removing participants, and counter-protesters tussling with armed officers.

Congressional candidate Samson Racioppi, one of several expected to speak told WCVB-TV that he didn't realize "how unplanned of an event it was going to be".

Earlier, thousands of leftist counter-protester marched through downtown Boston, chanting anti-Nazi slogans and waving signs decrying white nationalism.

Boston's Democratic mayor, Marty Walsh, and Massachusetts' Republican Governor, Charlie Baker, both said that extremist unrest would not be tolerated in the city touted as the birthplace of American liberty.

Dating back to 1634, Boston Common is the nation's oldest city park.

Across the globe, left-wing activists in Berlin prevented a neo-Nazi rally from marching to the prison where war criminal Rudolf Hess died 30 years ago.

Hess was convicted of war crimes at the Nuremburg trials in 1947 and died in Berlin's Spandau Prison in 1987.

Nazi sympathisers admire Hess because he never renounced his beliefs decades after the fall of the Third Reich.

Neo-Nazis had planned to march to the prison to commemorate the 30th anniversary of his death but were turned back by counter-demonstrators.

Police told CNN about 500 participants on each side turned out.

After World War II, strict laws in Germany banned Nazi symbols and hate speech, but under US law the rights of neo-Nazis, white supremacists and the Ku Klux Klan are protected, and these groups can hold public rallies and openly express their views.

Cities across the United States are gearing up for far-right rallies in the coming days, and officials have expressed concern about the potential for more violence.