Around 40 searches have taken place as part of a national operation into high-value museum thefts.
Nineteen people have been arrested following a series of dawn raids across England and Northern Ireland.
Officers have seized cars, cash, suspected stolen property and documents from addresses in Essex, Cambridgeshire, London, Sussex, the West Midlands and Northern Ireland.
Seventeen men and two women remain in custody at police stations in England and Northern Ireland.
Hundreds of officers from 26 police forces and the Serious Organised Crime Agency (SOCA) are involved in the operation and searches are ongoing at various locations.
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, aged 43, 46 and 59, have been arrested in Northern Ireland.
All of those arrested are being held on suspicion of conspiracy to burgle apart from the 54-year-old woman who was arrested on suspicion of perverting the course of justice and assisting an offender.
The investigation is in connection with six crimes over a four-month period at museums and auction houses across England.
Chinese artefacts and rhinoceros horn were stolen in six incidents - three at Durham Museum, Gorringes Auction House in East Sussex, Norwich Castle Museum in Norfolk and the Fitzwilliam Museum in Cambridge.
While much of the property has previously been recovered, several high-value items are still missing.
ACPO lead for serious organised crime, Chief Constable Mick Creedon 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.”
The Association of Chief Police Officers (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.
Anyone with any information should call Crimestoppers, anonymously, on 0800 555111.