Special counsel Robert Mueller on Friday submitted his long-awaited report into an explosive two-year investigation of Russian meddling in Donald Trump's 2016 election -- a probe the president denounces as a "witch hunt" and opponents say could fuel impeachment.

What the report says is confidential, but Attorney General Bill Barr wrote in a letter to Congress that he might be able to summarize its "principal conclusions" for Congress as early as this weekend.

The Mueller drama, filled with unprecedented allegations of collusion or even treason by a US president in league with Moscow, has dogged Trump since he took office following his surprise election defeat of Hillary Clinton.

Throughout, he has maintained that he is the victim of a "witch hunt," while Democratic opponents, who won control of the lower house of Congress last year, say Trump has yet to adequately explain his links to Russia.

Mueller, a Vietnam war veteran and former FBI director, worked in near total secrecy for two years. With his mission as special counsel wrapping up, it is now up to Barr, appointed by Trump, to decide how much of the report to make public.

Even before his report sees light, Robert Mueller's previous indictments and court filings have revealed much about the most shocking investigation of a presidential election in US history / © AFP

Public and political pressure for full disclosure is intense and Barr said he is "committed to as much transparency as possible."

There was one key piece of information already confirmed by the justice department, however: Mueller is not recommending any further indictments.

Over the course of his probe, Mueller charged three-dozen individuals and entities, including 25 Russians and six former Trump aides.

But the news that no more indictments are planned means potentially vulnerable figures close to the president, including his son Donald Trump Jr and powerful son-in-law Jared Kushner, will likely rest easier this weekend.

Trump himself made no comment from his Mar-a-Lago golf club resort in Florida, while he awaited the report. His spokeswoman Sarah Sanders said the White House now looks "forward to the process taking its course."

- Elaborate Russian meddling -

Even before the report sees light, Mueller's previous indictments and court filings have revealed much about the most shocking investigation of a presidential election in US history.

Robert Mueller has brought charges against six former Trump aides: pictured here clockwise from top left: Michael Cohen, Michael Flynn, Paul Manafort, Roger Stone, Rick Gates and George Papadopoulos / © AFP/File

These court documents described sustained efforts by Moscow to influence the 2016 vote and disrupt the country's democratic system.

Mueller described Russian government hackers and a social media troll farm working in a concerted effort to boost Trump over Clinton.

It was after seeing scores of unexplained contacts between the Trump campaign and Russians, that the FBI launched a probe into possible collusion. Trump then fired FBI chief James Comey and as a result the investigation was put in the hands of a special independent prosecutor -- Mueller.

Avoiding any leaks to the media -- a rare thing in Washington -- Mueller's crack team of lawyers brought charges against Trump associates Paul Manafort, Rick Gates, Michael Flynn, Michael Cohen, George Papadopoulos, and Roger Stone.

Five have been convicted of various crimes. Crucially, however, none have been charged with conspiracy to collude with the Russians.

- Trump against 'witch hunt' -

That fact has formed the basis of Trump's constant, loud assertions that "there was no collusion" and that he is the victim of a "hoax" run by Democrats.

In reality, legal experts say, Mueller may have dug up compromising material about Trump's Russia links, but not enough to stand up in a court of law.

That doesn't mean, however, that the issues will go away. Some could continue to be the subject of FBI counter-intelligence probes and some alleged crimes have already been picked up by regular federal prosecutors.

With Robert Mueller's mission as special counsel wrapping up, it is now up to Attormey General Bill Barr, to decide how much of his report to make public / © AFP/File

Among the many unanswered questions is the extent of Trump's business dealings with Russia, which include a previously undisclosed bid to build a Trump tower in Moscow, with talks continuing through 2016.

But Trump may be able to declare he has been vindicated from the start in saying Mueller would not demonstrate collusion.

The president had been attacking Mueller on these grounds just hours before the report dropped.

"For two years we've gone through this nonsense, because there's no collusion with Russia," he said. "People will not stand for it."

Mueller was also tasked with looking into whether Trump tried to obstruct justice by hampering the Russia probe. Trump had answer for that too.

"He (Trump) obstructed in fighting against the hoax. OK?" he said on Fox television.

- Full disclosure battle -

The battle will now rage over how much of the report can be seen and by whom.

Democrats called immediately for the contents to be released.

"It is imperative for Barr to make the full report public and provide its underlying documentation and findings to Congress," House Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer said in a joint statement.

Jerry Nadler, Democratic chairman of the House Judiciary Committee, which is to receive Barr's summary, said: "Transparency and the public interest demand nothing less. The need for public faith in the rule of law must be the priority."

The chairs of five other committees joined Nadler in a statement calling for the report to be released "without delay," as well as the underlying evidence uncovered during the investigation.

Those calls were echoed by Democratic presidential hopefuls including Senator Elizabeth Warren who urged: "Attorney General Barr -- release the Mueller report to the American public. Now."