'You made me a better man'

Immediate past US President Barack Obama wrote an emotional public letter thanking Americans and encouraging them to participate in "daily acts of citizenship."

"Before I leave my note for our 45th president, I wanted to say one final thank you for the honour of serving as your 44th," he wrote on Thursday.

"Because all that I've learned in my time in office, I've learned from you. You made me a better president, and you made me a better man."

Obama yesterday left the Oval Office for the last time before moving to the inauguration of his successor Donald Trump.

In the letter, he recalled low points -- like the massacre in a Charleston Church -- and high points -- like the legalisation of same-sex marriage -- during his presidency.

He also urged the American people, regardless of political affiliation, to commit themselves to the "joyous work of citizenship."

"I've seen you, the American people, in all your decency, determination, good humour and kindness. And in your daily acts of citizenship, I've seen our future unfolding.

"All of us, regardless of party, should throw ourselves into that work -- the joyous work of citizenship. Not just when there's an election, not just when our own narrow interest is at stake, but over the full span of a lifetime."

Obama said he would remain engaged in issues he cares about in his post-White House life.

"I'll be right there with you every step of the way," he wrote. "And when the arc of progress seems slow, remember: America is not the project of any one person. The single most powerful word in our democracy is the word 'We.' 'We the People.' 'We shall overcome.' Yes, we can.”

He also made some subtle digs at Trump in referring to issues on which they are divided.

He spoke of the success of his Obamacare health scheme, which Trump has vowed to undo. And he mentioned that even young children reminded us of our "obligations to care for refugees."

GITMO CLOSURE

Obama delivered an angry parting shot at Congress on Thursday, berating Republicans for blocking his efforts to close the military detention centre at Guantanamo Bay.

The facility "never should have been opened in the first place", Obama said in a scathing two-page letter.

"There is simply no justification beyond politics for the Congress' insistence on keeping the facility open," he added.

Trump has vowed not just to keep Guantanamo open, but to boost the number of terror suspects housed there -- even raising the prospect of US citizens being sent to the facility.

"We're going to load it up with some bad dudes, believe me, we're going to load it up," Trump said while campaigning last year.

On another occasion, he said "it would be fine" if US terror suspects were sent there for trial.

One of Obama's first acts as president in 2009 was to issue an executive order to shut the controversial jail within a year -- a move that once enjoyed bipartisan support.

But Republican opponents, sensing a political vulnerability, quickly reversed course after Obama took office and blocked moves to close Guantanamo.

COMMUTATIONS

Obama on Thursday commuted the sentences of 330 people, mostly drug offenders, a record number issued in a single day, on the eve of his departure from the White House.

It is Obama's second such measure this week, including his surprise decision to commute the sentence of transgender army private Chelsea Manning, jailed for 35 years for handing more than 700,000 classified US documents to WikiLeaks.

Originally set to be released in 2045, Manning will now walk free in May, reports AFP.

On Tuesday, the president pardoned another 64 people -- including a former general who was a key member of his first national security team -- and commuted the sentence of 208 prisoners in addition to Manning's.

Thursday's announcement raises Obama's record for commutations to a total of 1,715, more than any other US president. He has freed 568 inmates sentenced to life in prison.

Most of the beneficiaries were serving lengthy sentences for small and first-time drug offences. Obama has long called for correcting what he says is systematic injustice under a penal system now widely criticised for doling out excessive mandatory minimum sentences.

Obama, who favours alternative punishments for such offenders, repeatedly and unsuccessfully urged Congress to pass broad criminal justice reform.

More than 2.2 million people are currently behind bars in the US. They include legions of mentally ill and drug addicts, often from disadvantaged minority groups.