It was one of the biggest such releases of prisoners held in
Iraqi or American detention since US-led forces invaded the country three years ago to topple Saddam Hussein.
"I spent 16 months in jail without any specific reason. They only questioned me once, accusing me of funding terrorism," said one of more than 100 people freed in Baghdad.
The former prisoner said he had been arrested by Iraqi forces and then handed over to the US-run Abu Ghraib prison.
"I'm happy to return to my family," added Youssef Khidr, 38, looking exhausted.
Most of those in detention are believed held on suspicion of involvement in the insurgency.
The move by Mr Maliki appeared to be an attempt to shore up his own authority at a time when rivalries within his ruling Shi'ite
Alliance have cast doubt over his effectiveness.
State television, citing the Justice Ministry, said a total of
594 people had been freed across Iraq.
Many of those in detention -- estimated at more than 28,000 -- are from Saddam's once dominant Sunni community, which forms the backbone of an insurgency against the US-backed, Shi'ite-led government.
Mr Maliki, who has pledged to heal sectarian wounds and crush the insurgency, said in a televised statement the prisoner release would free those who had no clear evidence against them or had been detained mistakenly.
Initially, 500 people would be let out, he said, but did not give details.
It was not immediately clear how many of them were in Iraqi or US custody.
Mr Maliki had cited the release of those imprisoned without just cause as one of his priorities when his cabinet took office in May.
Such detentions, by Iraqi and US security forces, have been a major source of popular discontent.
But those considered "Saddam loyalists" or "terrorists" would not be freed.
A UN report last month said there were 28,700 detainees in Iraq, including 5,000 held by the Interior Ministry even though it should only detain people for short periods of time.
Mr Maliki's prisoner release came at a time when his fractious,
Shi'ite political Alliance is blocking his efforts to name new interior and defence ministers, who he hoped would lead his charge against insurgents and sectarian violence.
Meanwhile visiting Italian Foreign Minister Massimo D'Alema said he believed his country's troops would return home from Iraq by the end of this year.
Prime Minister Romano Prodi, who took office last month, has vowed to live up to campaign promises for a swift pull-out of Italy's military presence of around 2,600 troops. He has called the
Iraq war a "grave error".
"We think that by the end of this year the Italian military mission will end in Iraq," D'Alema said during a news conference with his Iraqi counterpart Hoshiyar Zebari, speaking through an Arabic translator.
Kidnapped people released
Meanwhile 10 of the 50 people kidnapped two days ago from a Baghdad street by armed gunmen have been released, security officials said.
"Ten of the 50 people were dropped by gunmen at the Palestine Street” (on Tuesday), an interior ministry official said, adding "three of them appeared to be wounded."
A defence ministry official also confirmed the release of the ten men.
"The released men were taken away immediately by a team of policemen," the interior ministry official said.
At least 50 people were snatched on Monday in a daring daylight raid by gunmen dressed in commando uniforms in Baghdad.