US President Donald Trump's longtime ally Roger Stone said on Thursday that he will plead not guilty to charges of lying to Congress about the release of stolen Democratic Party emails during the 2016 presidential campaign.

Stone was arrested in Florida on Friday before dawn, prompting Trump to lash out again about a "Witch Hunt" against him.

"I am falsely accused," Stone said outside the court in Ft Lauderdale, Florida, declaring himself "troubled by the political motivations of the prosecutors".

He also pledged his support for Trump - one of his "oldest friends" - saying, "I have made it clear that I will not testify against the president because I would have to bear false witness against him."

Stone was charged with seven criminal counts, including obstruction of an official proceeding, witness tampering and making false statements.

Special Counsel Robert Mueller said in court papers that Stone had advance knowledge of a plan by WikiLeaks to release the emails, which analysts say may have contributed to Trump's stunning defeat of Democrat Hillary Clinton.

Stone, a veteran political operative, is one of the closest Trump associates to be charged by Mueller, who is examining potential collusion between Trump"s campaign and Russia.

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"Greatest Witch Hunt in the History of our Country! NO COLLUSION!," Trump said on Twitter, using his most common slur for the Mueller probe.

The Kremlin has denied interfering with the 2016 election.

Profanity-laced messages

The indictment showed Stone using language evoking mob bosses - and even citing a "Godfather" movie - as he called an unnamed associate facing FBI inquiries "a rat" and "a stoolie" in a series of profanity-laced messages.

Stone was an early Trump backer whose reputation as an aggressive political operative and self-proclaimed "dirty trickster" dates back to the Watergate scandal of the 1970s when he was working for Richard Nixon. He has a tattoo of Nixon's face on his back.

His lawyer, Grant Smith, told Reuters that Stone would "vigorously" contest the charges.

"There was no collusion," Smith said. "He forgot to tell something to Congress and what it was was immaterial."

He appeared briefly in federal court in Fort Lauderdale, Florida, wearing blue jeans and a blue polo. A judge released him on a $250,000 bond and ordered him to limit his travel to South Florida, New York City and Washington.

WikiLeaks, which is referred to as "Organisation 1" in the indictment, did not respond to a request for comment.

Thirty-five people have pleaded guilty, been indicted or otherwise swept up in the Russia inquiry, which has clouded Trump's two-year-old presidency.

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Those people include former close associates of Trump such as his one-time lawyer Michael Cohen and former campaign chairman Paul Manafort as well as 12 Russian intelligence officers.

White House Press Secretary Sarah Sanders said the Stone indictment was unrelated to the president.

"The charges brought against Mr Stone have nothing to do with the president, has nothing to do with the White House," she told CNN. "The president did nothing wrong."

'Damaging information'

The charging documents included new details about the alleged activities of Trump aides, including an incident in which a senior campaign official "was directed to contact Stone about any additional releases and what other damaging information Organisation 1 had regarding the Clinton Campaign".

The indictment referred to an October 2016 email from the "high-ranking Trump Campaign official" asking Stone to inquire about future releases of emails by "Organisation 1".

Stone responded that "Organisation 1" would release "a load every week going forward".

The high-ranking official is Steve Bannon, Trump's former campaign chief, according to a person familiar with the matter. Bannon did not respond to a request for comment.

"The indictment was not unexpected, but it is still significant because it alleges coordination between the Trump Campaign and WikiLeaks," said Barbara McQuade, a former US Attorney for the Eastern District of Michigan.

Michael Zeldin, a former federal prosecutor, said the new details in the indictment were damaging politically to Trump but that it remained unclear if there was criminal exposure for anyone else in Trump's orbit.

He noted that Mueller made a point of portraying WikiLeaks as an organisation that has repeatedly been involved in posting stolen documents from US citizens.

"In Mueller's mind this is a campaign cavorting with the enemy," Zeldin said. "Politically there is a lot here that is pretty ugly. Legally it's not clear to me if there is evidence of people having colluded in a criminal sense."

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Trump's critics noted that the arrest showed Mueller's probe getting closer to the president himself.

"This is not some casual, low-level contributor to the Trump Campaign. This is someone who had been very active in Republican Party politics for a long time," Democratic US Senator Chris Coons told MSNBC in an interview.

Stone's ties to Trump go back decades.

He had urged Trump to run for president since 1988, was chairman of his presidential exploratory committee in 2000 and was a consultant when Trump considered running in 2012, according to Stone's 2017 book about Trump's campaign for the White House.

Stone briefly worked for the Trump campaign but left in August 2015. The campaign said it fired him after he tried to grab too much of the spotlight while Stone insisted that he quit.