All of the targets are frequently disparaged by right-wing critics and Donald Trump.

Police intercepted suspected pipe bombs sent to former US President Barack Obama, former presidential candidate Hillary Clinton, and other high-profile Democrats on Wednesday, in what New York officials described as an act of terrorism.

The CNN bureau in New York also received a device looking like a pipe bomb, leading police to evacuate the building, as did the office of New York Governor Andrew Cuomo, a Democrat. CNN reported that Eric Holder, who was US Attorney General under former President Barack Obama, was 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.

With the country deeply polarized under President Donald Trump, the packages brought a new level of tension to November 6 political contests that will decide whether Democrats can challenge the majorities now held by Trump's Republicans in Congress.

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 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 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."

Packages Recovered

The package to Clinton was found late on Tuesday while the one addressed to Obama was recovered early on Wednesday, both during routine off-site mail screenings, the Secret Service said. Obama and Clinton 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 September 11, 2001, anthrax spores were mailed to news media offices and two US senators, killing five people and infecting 17 others.

CNN is a frequent target of 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.

Brennan has been an outspoken critic of 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 mailed devices had also been intercepted, apparently with the return address of US Representative Debbie Wasserman Schultz, a former Democratic Party national chairwoman.

Wasserman Schultz's office was evacuated after a suspicious package arrived in the mail, WFOR-TV in Miami reported, citing Sunrise police. CNN said the package was meant for Holder but sent to the wrong address and returned to Wasserman Schultz's office.

A source familiar with the matter earlier said a suspicious package addressed to the White House had also been intercepted, but soon after said that information was wrong.