City of London Police were asked to work with the Food Standards Agency (FSA) as part of its inquiry into the scandal. It reviewed evidence from law enforcement agencies in Europe and the UK, as well as from the FSA.
The force launched an investigation in May and said it made the arrests "during the initial stages" of the inquiry. Officers also carried out searches at businesses and homes in the UK.
Detective Chief Superintendent Oliver Shaw, from the City of London Police, said:
This is an extremely complex investigation covering a number of jurisdictions and a variety of businesses.
We are working closely with police forces, other law enforcement agencies and regulators to determine whether horsemeat being used in a range of meat products was deliberate and coordinated criminal activity.
More top news
The party's deputy-leader is the sole candidate in the race to succeed Gerry Adams.
A Met Office yellow "be aware" warning for ice is in place across much of Scotland, Northern Ireland and northern England.
Nick Boles warned the PM it was "time to raise your game" and claimed her government "constantly disappoints" in a tweet.