They were held on suspicion of offences including arms trafficking, criminal association, abuse of power, and the falsification of documents, said a statement from the prosecutor's office.

The arrests come more than a year after a thieves broke into a Portuguese army arms depot in Tancos, central Portugal, where security cameras had been out of order for two years.

In all, they got away with 44 anti-tank rockets, 150 hand grenades, 18 tear gas grenades, and 1,450 nine-millimetre cartridges.

The theft happened in June 2017, and it was only an anonymous tip-off in October that allowed police to recover most of the haul, hidden on waste ground a few dozen kilometres from Tancos.

The cartridges are still missing, however.

Among those in custody, according to local media reports, were Luis Vieira, head of the military police, three gendarmes and a civilian.

Police raided sites in the capital and across the country, from Porto in the north, Santarem in central Portugal, and Algarve in the south, said prosecutors.

Investigators had been working on the theory that the theft was an inside job, as those responsible seemed to know the arms depot well.