The Financial Times reported that the seven are allied to three companies.
Two are believed to be small family-owned businesses while a third is a larger private company.
All the suspects have been released on bail.
The Office of Fair Trading told the FT it “has opened a criminal investigation into suspected cartel activity in the supply of products to the construction industry.
“As part of the investigation searches were carried out last week at a number of locations across the UK.
“Seven individuals were also arrested in the Midlands. No assumption should be made at this stage that any criminal offences have been committed.”
The current arrests are not related to the OFT’s major investigation into cover pricing which saw fines of £130m imposed on contractors in 2009.
The fines were subsequently cut following a series of successful appeals by contractors.