Spanish police have seized what is thought to be a national record haul of cocaine from a ship in the Atlantic Ocean.

Between 30,000 to 45,000kg were found when the Civil Guard intercepted a freighter in international waters, the body's main union, the AUGC, announced. It called the move a "historic blow to drug trafficking".

The vessel was intercepted off Spain's Canary Islands on Friday and around 20 people were arrested, the AUGC told the AFP news agency. It had travelled from Sierra Leona and was on its way to Libya.

The Civil Guard has declined to give details of the investigation for legal reasons.

Interior Minister Fernando Grande-Marlaska told reporters in Madrid that the seizure was "one of the biggest, not only nationally but internationally".

The Civil Guard shared a photograph on X showing the drugs stuffed into the hold of the intercepted vessel.

"Today history is being written in the Maritime Service of the Civil Guard," it wrote.

"Intercepted in international waters the largest known seizure: between 30,000 and 45,000 kg of cocaine on board a freighter."

While the boat was headed to Libya, AFP reported that the pattern of previous operations suggests that it was due to offload the drugs onto smaller vessels for distribution in Europe.

In January, Spanish authorities made its biggest seizure of cocaine at sea from a ship that was carrying almost 10 tonnes.