US police have intercepted suspected devices sent to former president Barack Obama, former presidential candidate Hillary Clinton, and other high-profile Democrats, in what New York officials described as an act of terrorism.

With the country deeply polarised, the packages brought a new level of tension to elections on 6 November that will decide whether Democrats can challenge the majorities held by Republicans in Congress.

The CNN bureau in New York also received a device looking like a pipe bomb, leading police to evacuate the building.

Eric Holder, who was US attorney general under Mr Obama, and Democratic representative Maxine Waters were also among those targeted.

"This clearly is an act of terror attempting to undermine our free press and leaders of this country through acts of violence," New York Mayor Bill de Blasio told a news conference.

US President Donald Trump condemned what he called "despicable acts" and vowed to bring those responsible to justice.

"In these times, we have to unify, we have to come together, and send one very clear, strong, unmistakable message that acts or threats of political violence of any kind have no place in the United States of America,"

Mr Trump said at the White House. "We're extremely angry, upset and unhappy about what we witnessed this morning, and we will get to the bottom of it," he added.

A similar pipe bomb was delivered earlier this week to the home of George Soros, a major Democratic Party donor.

There has been no claim of responsibility.

All of the targets are frequently disparaged by right-wing critics and by President Trump, whose spokeswoman condemned the acts.

"Our condemnation of these despicable acts certainly includes threats made to CNN as well as current or former public servants.

"These cowardly acts are unacceptable and won't be tolerated," Press Secretary Sarah Huckabee Sanders said on Twitter.

Alexander Soros, the son of George Soros, said in an opinion piece published by The New York Times that his father had long faced verbal criticism and threats over his involvement in politics, "but something changed in 2016" when Mr Trump was elected.

"Before that, the vitriol he faced was largely confined to the extremist fringes, among white supremacists and nationalists who sought to undermine the very foundations of democracy.

"But with Donald Trump's presidential campaign, things got worse," Alexander Soros wrote.

He placed direct responsibility with those who sent the devices, but added: "I cannot see it divorced from the new normal of political demonisation that plagues us today."

The package sent to Mrs Clinton was found late yesterday, while the one addressed to Mr Obama was recovered early today, both during routine off-site mail screenings, the Secret Service said.

Mrs Clinton or Mr Obama were not at risk, they said.

Police intercepted what appeared to be a live explosive device and an envelope containing white powder at the Time Warner Center, where the CNN newsroom in New York is based, Police Commissioner James O'Neill told the news conference.

Experts were analysing whether the powder was a dangerous substance, he said.

In the weeks following the attacks of 11 September 2001, anthrax spores were sent in the post to news media offices and two US senators, killing five people and infecting 17 others.

CNN is a frequent target of President Trump, who routinely calls the news media the "enemy of the people" and belittles critical coverage as "fake news."

Officials said the package received by CNN was addressed to former CIA director John Brennan, a periodic guest commentator on the cable network.

Mr Brennan has been an outspoken critic of Mr Trump, calling the president's performance during a July joint news conference with Russian President Vladimir Putin in Helsinki "nothing short of treasonous."

Media reported that other devices had also been intercepted, apparently with the return address of US representative Debbie Wasserman Schultz, a former Democratic Party national chairperson.

Ms Wasserman Schultz's office was evacuated after a suspicious package arrived in the mail, WFOR-TV in Miami reported, citing police.

CNN said the package was meant for Mr Holder but sent to the wrong address and returned to Ms Wasserman Schultz's office.