Paul Manafort, a former campaign manager for President Donald Trump, and his business partner pleaded not guilty Monday to charges of money laundering and conspiracy against the United States, the most serious step yet in a special counsel investigation into Russian links with Trump's 2016 White House campaign.

Manafort and Rick Gates entered not guilty pleas in a Washington, D.C., court on Monday afternoon.

They are the first announced charges from special counsel Robert Mueller's investigation, which is also examining possible collusion between members of Trump's campaign and transition teams and foreign governments. Mueller has been given the authority to prosecute any alleged crimes that result from the course of the investigation.

A third former Trump adviser, George Papadopoulos, pleaded guilty in early October to lying to the FBI, it was also announced.

An indictment approved by a grand jury alleges Manafort and Gates conspired to defraud the United States "from in or about and between 2006 and 2017."

Paul Manafort, right, is seen with Donald Trump on July 21, 2016, at the Republican National Convention in Cleveland. Manafort alleges Special Counsel Robert Mueller's office's investigation into possible collusion between the Trump campaign and Russia has exceeded its legal authority. (Rick Wilking/Reuters)

The indictment says more than $75 million US flowed through Manafort's and Gates's offshore accounts. Manafort, the indictment alleges, laundered more than $18 million.

"The indictment contains 12 counts: conspiracy against the United States, conspiracy to launder money, unregistered agent of a foreign principal, false and misleading FARA statements, false statements, and seven counts of failure to file reports of foreign bank and financial accounts," the counsel said in a statement, referring to the Foreign Agents Registration Act.

Gates, 45, served as his deputy in the Trump campaign. Manafort and Gates are accused of generating tens of millions of dollars in income from work for Ukrainian political parties and leaders.

Manafort was indicted on nine counts and Gates was indicted on eight counts.

Manafort was released Monday on home confinement after posting a $10 million bond. Gates was released on a $5 million bond.

Sorry, but this is years ago, before Paul Manafort was part of the Trump campaign. But why aren't Crooked Hillary & the Dems the focus????? —@realDonaldTrump

....Also, there is NO COLLUSION! —@realDonaldTrump

If convicted, Manafort potentially would face up to 80 years in prison, according to a review of the federal charges and the relevant statutes by The Associated Press, while Gates would face up to 70 years.

Among other things, Mueller has been investigating Manafort's financial and real estate dealings and his prior work for that political group, the Party of Regions, which backed former Ukrainian leader Viktor Yanukovich, sources have told Reuters. In July, FBI agents raided Manafort's Virginia home.

Kevin Downing, attorney for Manafort, angrily rejected the charges in a statement outside the federal courthouse in Washington. Downing said the charges related to his client's offshore money transfers are "ridiculous."

Manafort allegations 'years ago': Trump

Manafort, 68, served the Trump campaign from June to August of 2016 before resigning amid reports that he might have received millions in illegal payments from a pro-Russian political party in Ukraine.

Investigations into Russian links with the Trump campaign, including Mueller's and probes by several congressional panels, have dogged Trump's presidency since the Republican took office in January and widened the partisan rift between Republicans and Democrats.

Rick Gates, a campaign aide to Republican presidential candidate Trump, surrendered to federal authorities Monday. He, too, pleaded not guilty to the charges against him. (Evan Vucci/Associated Press)

U.S. intelligence agencies concluded in January that Russia interfered in the election to try to help Trump defeat Democrat Hillary Clinton by hacking and releasing embarrassing emails and disseminating propaganda via social media to discredit her.

Trump has denied the allegations and called the probe "a witch hunt."

The president has also sought to shift attention to a uranium deal with a Russian firm that took place during Clinton's time as secretary of state and a dossier containing allegations about Trump and Russia that the Democrats were in possession of during the campaign.

Trump again made Clinton accusations in his first tweet since the Manafort indictment was announced. He characterized the allegations Manafort faced as predating the Trump campaign, although the indictment states the activities continued into 2017.

Sarah Huckabee Sanders, the White House press secretary, also rejected any connection to Trump's campaign in a briefing with reporters. When asked whether the president would consider pardons of any individuals found guilty in the criminal investigation, she said the administration would "let the process play" through.

Sanders said she didn't believe that the indictment was a commentary on the thoroughness, or lack thereof, of the vetting process of Trump campaign officials, or the president's judgment, given that the alleged activities were over a number of years.

Trump adviser sought Clinton 'dirt' during campaign

Mueller, a former director of the Federal Bureau of Investigation, has been looking into possible links between Trump aides and foreign governments, as well as potential money laundering, tax evasion and other financial crimes, according to sources familiar with the probe.

He also is exploring whether Trump or his aides have tried to obstruct the investigation, and on Monday, the special counsel's office announced that Papadopolous pleaded guilty on Oct. 5 to making false statements to FBI agents.

George Papadopolous, circled, is shown in a March 31, 2016, image released by the Trump campaign on the candidate's Instagram account. (Donald Trump/Instagram)

Papadopolous, a Chicago native and international energy lawyer, was part of Trump's advisory team during the 2016 presidential campaign.

The special counsel said Papadopoulos told FBI agents he had been in contact with an unnamed foreign "professor" who claimed to have "dirt" on Clinton in the form of "thousands of emails," and that Papadopolous claimed such contacts occurred before he joined Trump's campaign.

But the prosecutor said Papadopolous in fact did not meet the professor until after he joined Trump's campaign.

Court documents from the special counsel's office outline the charges and facts of Papadopoulos's case. They also look at sentencing, saying "your client's estimated sentencing guideline range is zero to six months imprisonment" along with a fine of between $500 and $9,500.

Trump has not yet commented on the Papadopolous plea but Sanders sought to diminish his role in the campaign, calling him a member of a volunteer advisory committee and that his plea relates to "his failure to tell the truth" and not campaign activities.

However in court documents released Monday, an unidentified campaign official is said to have advised Papadopoulos around May 2016 that Trump himself "is not doing these trips," but that "it should be someone low level in the campaign so as not to send any signal."

Sanders said "she was not aware of that conversation" and refused to comment.

Conservative allies attack Mueller

Mueller was appointed to lead the investigation a week after Trump's May 9 firing of FBI director James Comey, who was heading a federal probe into possible collusion with Russia.

AS IT HAPPENS | What Robert Mueller can and can't do as special counsel in the Trump-Russia probe

Trump initially said he fired Comey because his leadership of the FBI was inadequate. In a later interview with NBC, he cited "this Russia thing" as his reason.

Special counsel Robert Mueller is investigating whether Trump campaign officials colluded with Russia to meddle in the 2016 U.S. presidential election. (J. Scott Applewhite/Associated Press)

The White House said in the summer that Trump had no intention of firing Mueller even though he questioned his impartiality. The possibility of arrests and indictments spurred some of Trump's allies in conservative media outlets to call for Mueller's firing in recent days.

Democratic Senate leader Chuck Schumer on Monday called for the Trump administration to avoid interfering with the special prosecutor's probe.

"The rule of law is paramount in America and the investigation must be allowed to proceed unimpeded," said Schumer. "The president must not, under any circumstances, interfere with the special counsel's work in any way."