More than 10,000 demonstrators chanting anti-Nazi slogans forced a small group of conservatives to cut short a "free speech rally" in Boston, in a repudiation of racism a week after deadly white supremacist violence in Charlottesville.

About 25 conservative rally-goers gathered on Saturday at Boston Common park, but abruptly left about 90 minutes after the event started.

Al Jazeera's Shihab Rattansi, reporting from Boston, said thousands jeered as the group was "escorted out" the park by police officers.

OPINION: Charlottesville is America everywhere

According to reports, an estimated 15,000 people showed up at the counterprotest.

They chanted anti-Nazi and anti-fascism slogans, and waved signs that said: "Love your neighbor", ''Resist fascism" and "Hate never made US great".

Others carried a large banner that read: "SMASH WHITE SUPREMACY".

One of the planned speakers of the "free speech rally" was quoted by AP news agency as saying the conservatives' event "fell apart".

A number of far-right activists had been asked to speak at the event, but some dropped out, according to local media.

Far-right groups often use the guise of free speech to incite hatred.

Police had deployed 500 officers - some in uniform, others undercover - to keep the two groups apart.

Many had feared violence after an August 12 rally in Charlottesville turned deadly. Then, a white supremacist rammed through crowds of people, killing 32-year-old anti-racist protester Heather Heyer and injuring dozens.

'Spewing hate'

The Boston Free Speech Coalition, which organised the event, said it has nothing to do with racism and is not affiliated with the Charlottesville rally organisers.

Samson Racioppi, a Libertarian Party congressional candidate from the US state of Massachusetts, was slated to speak on the "free speech" stage.

He told WCVB-TV he did not realise "how unplanned of an event it was going to be".

Earlier, he said that when free speech is "suppressed", the only recourse "extremists will have remaining is [sic] terrorism and violence".

Kyle Chapman, who described himself on Facebook as a "proud American nationalist," had also announced his attendance.

Mayor Marty Walsh had pointed out that some of those invited to speak "spew hate".

Saturday's showdown was mostly peaceful, and after demonstrators dispersed, a picnic atmosphere took over with stragglers tossing beach balls, banging on bongo drums and playing reggae music.

"I came out today to show support for the black community and for all minority communities," said Rockeem Robinson, 21, a youth worker from Cambridge.

He said he was not concerned about his personal safety because he felt more support on his side.

Katie Griffiths, 48, a social worker also from Cambridge, who works with members of poor and minority communities, said she finds the hate and violence happening "very scary".

"I see poor people and people of colour being scapegoated," she said. "Unlearned lessons can be repeated."

Events were also planned around the country, in cities including Atlanta, Dallas and New Orleans.

Social media users celebrated developments in Boston as a feat against rising racism and hate crimes in the United States.

However, President Donald Trump berated counterprotesters, accusing them of being "anti-police agitators".

Trump, whose response to white-supremacist-led violence in Charlottesville has been heavily criticised, tweeted: "Looks like many anti-police agitators in Boston. Police are looking tough and smart! Thank you."

The president later tweeted: "I want to applaud the many protesters in Boston who are speaking out against bigotry and hate. Our country will soon come together as one!"

It was unclear which set of "protesters" Trump was referring to.

Here are some reactions and images of the event posted to social media:

The hate speech rally in Boston has about 100 white supremacists and TWENTY THOUSAND peaceful protestors...pic.twitter.com/bjaRqVWN4Q — Mikel Jollett (@Mikel_Jollett) August 19, 2017

Police were getting cheers from the counter-protesters in Boston. https://t.co/Xx9nLksNqz — John Rosevear (@john__rosevear) August 19, 2017

Props to the woman who carried this in Boston. Wanted to give it a signal boost on Twitter. pic.twitter.com/lxliNL0dEo — Kristi Winters (@KWintie) August 19, 2017

if the Alt Right had any sense, they would have cancelled the Boston rally. Instead, they finally get to see how hated they really are. — New York City Antifa (@NYCAntifa) August 19, 2017

More anti-nazi protesters showed up in Boston today than folks at Trump's inauguration — Alexandra Halaby (@iskandrah) August 19, 2017

Happy to report that there were thousands and thousands of "very fine people" rallying in Boston today and they were ALL ON ONE SIDE — Seth Abramson (@SethAbramson) August 19, 2017

45 minutes into the planned rally and the 'Free Speech' protestors are leaving. well done, #Boston pic.twitter.com/jNePGtqPew — Jamie Church (@jlchurch) August 19, 2017

There's a group of witches counter-protesting the Boston alt-right rally and I'm ready to become Supreme and lead a KC chapter of this coven pic.twitter.com/jiJscjoZpc — Pat (╯°□°）╯︵ ┻━┻ (@PatsHoppedUp) August 19, 2017