Bradley Manning will send a personal plea to Barack Obama next week for a presidential pardon after he was sentenced on Wednesday to 35 years in prison for passing hundreds of thousands of classified military documents to WikiLeaks.

The sentence was more severe than many observers expected, and is much longer than any punishment given to previous US government officials who have leaked information to the media.

Manning showed no emotion, neither when the sentence was delivered, nor after being escorted into a side room, where his lawyers and members of his family were waiting, some of them in tears.

"Everyone in his defence team was emotional, including myself," his lawyer, David Coombs, told the Guardian. "The only person that wasn't emotional was Brad. He looked to us and said: 'It's OK. I'm going to move forward and I'm going to be all right'."

Coombs told a press conference that next week he will formally submit the request for a pardon, "or at the very least commute his sentence to time served". That request will contain a personal appeal from Manning to Obama, which his lawyer read out.

"When I chose to disclose classified information, I did so out of a love to my country and a sense of duty to others," Manning will tell Obama. "If you deny my request for a pardon, I will serve my time knowing that sometimes you have to pay a heavy price to live in a free society."

Coombs said the military's decision to seek a charge of aiding enemy – which ultimately failed – was placed amid a "government-wide crackdown" on journalists and whistleblowers that should alarm those who care about a free press.

"The case of the United States v Bradley Manning is a watershed movement in history for the freedom of the press," he said.

The 25-year-old soldier was convicted last month of leaking more than 700,000 classified documents and video. The disclosures amounted to the biggest leak in US military history.

He was found guilty of 20 counts, six of them under the Espionage Act, but was acquitted of the most serious charge of "aiding the enemy".

A protracted legal process that started in May 2010, when Manning was arrested while stationed in Iraq, was over in less than two minutes on Wednesday morning.

The military judge presiding over the court martial, Colonel Denise Lind, walked into the courtroom at Fort Meade military base at 10.15am, dealt with some court admin, asked Manning to stand, then told him he was sentenced to 35 years.

Speaking a clipped tone, Lind told the soldier he would be reduced in rank to the lowest grade of army private, see his pay and allowance forfeited, be dishonourably discharged from the military and "confined for 35 years".

He will now be transferred to military custody in Fort Leavenworth, Kansas. Amid some confusion over the military rules for parole, his legal team said that taking into account the time he has already served, he will be eligible for parole in around seven years. He has to serve a minimum of a third of his sentence, and at the very earliest, could be released under parole soon as 2021.

A total of 1,294 days – more than three years – are automatically deducted from Manning's sentence.

That includes the time already spent in military custody since May 2010, plus 112 days that is being taken off the sentence as part of a pre-trial ruling in which Lind compensated Manning for the excessively harsh treatment he endured at the Quantico marine base in Virginia.

He can earn 120 days per year off his sentence for good behaviour and job performance.

After the judge left the court, Manning was quickly ushered out by guards. A handful of supporters were heard to shout "We'll keep fighting for you Bradley" and "You're our hero".

Manning had faced a maximum possible sentence of 90 years, although few legal experts expected he would receive anything near that amount.

Prosecutors had asked the judge to jail Manning for at least 60 years. But observers who closely followed the Manning trial regarded a sentence of around 20 or 25 years as something of a benchmark.

If the prosecution had ended the trial in February, when Manning pleaded guilty to some of the counts against him, his maximum jail term would have been 20 years.

In mitigation, the soldier's defence team said he should receive no more than 25 years – the period of time after which many of the materials he released would have been automatically declassified.

The sentence was immediately criticised by press freedom groups and civil liberty campaigners.

Ben Wizner, director of the American Civil Liberties Union's Speech, Privacy & Technology Project, said: "When a soldier who shared information with the press and public is punished far more harshly than others who tortured prisoners and killed civilians, something is seriously wrong with our justice system.

"A legal system that doesn't distinguish between leaks to the press in the public interest and treason against the nation will not only produce unjust results, but will deprive the public of critical information that is necessary for democratic accountability."

He added: "This is a sad day for Bradley Manning, but it's also a sad day for all Americans who depend on brave whistleblowers and a free press for a fully informed public debate."

Daniel Ellsberg, who faced charges under the Espionage Act for leaking the Pentagon Papers documenting the Vietnam war, said Manning "doesn't deserve to spend another day in jail".

"There are some that will not be deterred even by prospect of life in prison – I think that Manning was one of those," he said. "I think that Edward Snowden is another – he knows he faced the prospect of life in prison or even assassination … This is an effort to minimise truth-telling."

"This is unprecedented," said Liza Goitein, who co-directs the Brennan Center for Justice's Liberty and National Security Program. "It is dramatically longer than the longest sentence ever served for disclosing classified information to the media, which was two years."

WikiLeaks, however, hailed the sentence as a "significant strategic victory". In a statement posted on the organisation's website, founder Julian Assange called the trial "an affront to basic concepts of western justice".

Assange said "the Obama administration is demonstrating that there is no place in its system for people of conscience and principle" and warned "there will be a thousand more Bradley Mannings".

Military lawyers specifically urged Lind to jail Manning for the "majority of his remaining life" to deter potential future leakers from passing journalists documents on such a scale.

Captain Joe Morrow, a lawyer for the government, told the judge on Monday that is was Lind's responsibility to ensure the military "never see" another leak on the scale of Manning's releases. "This court must send a message to any soldier contemplating stealing classified information," he told her.

Manning's sentence vastly outweighs any previous sentence given to a US leaker who has passed information to a journalistic outlet, although the nature and scale of Manning's disclosures was unprecedented.

Government workers successfully convicted for unauthorised disclosures in recent years include Shamai Leibowitz, an FBI translator who was sentenced to 20 months after passing secret transcripts to a blogger, and John Kiriakou, a former CIA officer who was sentenced to 30 months after pleaded guilty last year to disclosing information about the waterboarding of terror suspects.

In 2011, another government whistleblower, Thomas Drake, who shared information about National Security Agency technologies with a Baltimore Sun reporter, was sentenced to 240 hours of community service after a plea bargain.

The sentence marks the end of a long journey for the soldier, which began in late 2009, when he was stationed in the Iraq desert as an intelligence analyst. Disillusioned over the war, Manning, from Oklahoma, began downloading documents from classified computers onto CDs.

Manning passed 250,000 State Department cables and 470,000 Iraq and Afghanistan battlefield logs to WikiLeaks, as well as files pertaining to detainees held at Guantanamo Bay, and video of a 2007 attack by a US helicopter gunship in Baghdad that killed a dozen people, including two Reuters journalists.

WikiLeaks published some material on its site and also shared documents with a consortium of news organisations, led by the Guardian.

Manning was arrested in May 2010, after Adrian Lamo, a computer hacker who conversed with the soldier in online chats, shopped him to the FBI.

He pleaded guilty to some of the charges in February, and probably achieved some reduction of his sentence when he told the judge earlier this month that he regretted his actions and was sorry that his leaks "hurt the United States".

Unlike civilian courts, where there are federal tariffs or sentencing guidelines, the sentence in a military court is subject to the sole discretion of the judge.

The case will now be automatically referred to the army court of criminal Appeals, the first step in what could become a protracted legal battle that could potentially culminate at the US supreme court.