Three men have been arrested in Belfast in connection with raids on museums and auction houses in England in which artefacts worth millions of pounds were stolen.
Nineteen people have been arrested in total across the UK.
The men arrested in Belfast are aged 43, 46 and 59.
Seventeen men and two women are currently in custody awaiting questioning following dawn swoops.
Hundreds of officers from 26 police forces and the Serious Organised Crime Agency (Soca) are involved in the operation and searches are continuing at various locations.
Warrants were executed in London, Sussex, the West Midlands, Cambridgeshire, Essex and Northern Ireland.
The operation follows a spate of crimes which took place at museums and auction houses across England over a four-month period last year.
Chinese artefacts and a rhinoceros horn were stolen in six incidents - three at Durham Museum, one at Gorringes Auction House in East Sussex and one each at Norwich Castle Museum and the Fitzwilliam Museum in Cambridge.
Five men, aged 20, 31, 35, 53 and 54, and two women, aged 28 and 54, were arrested in London.
Four men, aged 24, 41, 44 and 56, were arrested in Cambridgeshire.
Two men, aged 28 and 46, were arrested in Essex.
A 60-year-old man was arrested in Sussex, a 32-year-old man was arrested in the West Midlands and a 67-year-old man was arrested in Nottingham.
Three men have been arrested in Northern Ireland.
Cambridgeshire Police, who are leading the operation, said all of those arrested were being held on suspicion of conspiracy to burgle except the 54-year-old woman, who was arrested on suspicion of perverting the course of justice and assisting an offender.
So far, eight people have been convicted and jailed for their roles in the break-ins.
Much of the stolen property has since been recovered but several valuable items are still missing.
Speaking after a 15-year-old boy and three men were convicted of conspiracy to burgle following the raid on the University of Cambridge’s Fitzwilliam Museum, Cambridgeshire Police acknowledged they were unlikely to have caught everybody involved.
Eighteen ancient Chinese jade artefacts - which some experts said could be worth £18 million - were stolen after the gang evaded museum security.
Defence counsel told the court “others higher up the chain”, who have not been identified, recruited the men to target the jade exhibits.
Cambridge Crown Court heard it was unlikely the men knew the true value of the items, which it is thought were sold to rich Chinese collectors.
Chief Constable Mick Creedon, the Association of Chief Police Officers (Acpo) lead for serious organised crime, said: “Today’s operation follows a long and complex pan-European investigation involving officers from 26 police forces and the Serious Organised Crime Agency.
“The series of burglaries last year had a profound effect on museums and similar institutions and we are committed to bringing all those who were involved in the conspiracy to justice.
“Many of the stolen Chinese artefacts are still outstanding and a substantial reward remains on offer for information which leads to the safe return of those priceless items.”
Acpo has set up a working group to crack down on the emerging issue of heritage crime.
The group brings together police officers, English Heritage and other partners in the arts and museum sector to share intelligence and crime prevention advice.