Awakening group leaders Khaled al Qaisi, centre and Faris Abdel Hassan, right
Iraq's Awakening groups have been given much of the credit for reducing the level of violence in the country.
But the arrest of a prominent leader of one of the groups may signal a wavering of loyalty to the government among some of the fighters, says the BBC's Hugh Sykes in Baghdad.
The Awakening fighters in the Fadhil neighbourhood of Baghdad look as if they are in control.
Their deputy leader, Khaled al Qaisi, strode into Fadhil alongside Faris Abdel Hassan, who runs a comparable group in the neighbouring Shia district of Abu Saifain.
They both operate checkpoints and foot patrols.
Faris arrived in a pickup truck, two men with rifles standing on the back.
He and Khaled greeted each other with kisses on the cheek and arms across the shoulder.
Security is good in Fadhil because of the 'Sahwa' or Awakening
They declared they were "brothers", committed to the unity of Iraq - "one heart, one people," Khaled promised.
"Whatever he says, I agree," said Faris.
When I walked around Fadhil early on Saturday afternoon, the mood was cheerful and friendly.
Shops were open, children were playing, and men were playing backgammon in a small café.
Behind an alluring display of bananas, cucumbers and shiny red apples, Ahmad - cigarette in one hand - told me security was good.
"Why?" I asked.
"Sahwa," he replied - the Awakening.
Two years ago, Fadhil was a front line in fighting between al-Qaeda in Iraq and militias in the Shia neighbourhoods that encircle it.
Many buildings are scarred with the impact of gunfire.
About an hour after I left Fadhil, the mood there was no longer cheerful and friendly.
Local fighters and the Iraqi army were shooting at each other - Iraqi TV news showed men in uniform running through narrow alleyways, sometimes ducking for cover.
There were snipers on rooftops.
An American helicopter growled overhead.
Iraqi security forces - supported by US troops - had arrested Adil al Mashadani, the leader of the Fadhil Awakening group.
His men responded with fury, indignation - and gunfire.
Five Iraqi soldiers - including an officer, according to one report - were taken hostage. Their fate is still unknown.
A photograph published on an official website shows Mr Mashadani standing in an office of the Ministry of the Interior, his wrists cuffed in front of him. He is wearing a beige polo shirt - with his name scrawled in English on white tape stuck across his chest.
He faces numerous charges.
A press release from the Multi-National Force in Iraq says he is suspected of leading a cell that has attacked and killed Iraqi security forces with IEDs - roadside bombs.
They also believe he operated mortar or rocket teams, and "extorted bribes in excess of $160,000 (£113,000) a month from the citizens of Fadhil", as the press release puts it.
The Iraqi authorities have made another serious accusation against Mr Mashadani - that he maintained links with remnants of the former regime of Saddam Hussein.
The Centre for Imposing Law on Baghdad - set up under the "surge" of US troops in 2007 - says Mr Mashadani was running a new military wing of the old Ba'ath Party.
Awakening groups helped halt the progress of al-Qaeda in Iraq
The Ba'ath Party is proscribed under the new Iraqi constitution.
If this is true, it is a profoundly worrying development.
But it would not be a surprise.
Numerous Iraqis have told me they do not believe the new peace here is permanent.
They say many members of the old regime remain angry that they lost their status, their power and their income when the armed forces and the Ba'ath Party were disbanded in the early days of the occupation of Iraq - angry, and determined to get their power back.
One former Ba'athist asked me rhetorically four years ago, when the insurgency was accelerating:"What should they do? Sit in their kitchens with their wives? Do you expect them to sing and dance?"
"They can do anything and they can do everything," he added.
A widely expressed fear here is that these "remnants" have been only temporarily suppressed by the surge - and that they are "sitting on their hands" waiting to resume their activities when American forces leave.
The arrest of Mr Mashadani - and 14 of his men - is also a potential threat to the crucial, but fragile and mutually suspicious relationship between the Iraqi government and the Awakening movement.
The BBC in Baghdad has spoken to three other Awakening leaders in the city about the arrest of Adil al Mashadani.
Two said that no one should be above the law, that anyone accused of wrongdoing should face justice; although, in the opinion of one of them, the arrest was "provocative".
The third said Mr Mashadani should not have been detained, and that he should be praised for establishing security in Fadhil - but he added that it would not affect his commitment to using his Awakening men to keep the peace in his own neighbourhood.
Two years ago, Fadhil was a violent front line for al-Qaeda
On an Arabic TV channel, the Awakening leader in Anbar province west of Baghdad - where the movement was born - urged the government to integrate more of the movement's members into the armed forces.
He said there was still a risk that they could be recruited back to al-Qaeda in Iraq.
And the leader of the Awakening in Baquba - capital of Diyala province, where al-Qaeda still have active units - warned: "The government must remember that without the Sahwa there would not have been any security in Iraq."
In Fadhil, before the arrest operation on Saturday, the deputy Awakening leader Khaled al Qaisi told me he was also concerned that his men could be tempted back to al-Qaeda - mainly because they had not been paid for two months.
The Iraqi army and police say they are now in control of Fadhil, and that Khaled al Qaisi and his men have been disarmed.
They also say that local residents willingly helped them with their operation against Mr Mashadani.