Eleven people have been arrested at locations around the country on suspicion of tax relief fraud.
It follows raids by HM Revenue and Customs (HMRC) officers at a number of premises on Tuesday.
The BBC understands warrants were executed in Norwich at the companies Green Jellyfish and Kirby and Haslam.
Both companies said they had “nothing to hide”.
According to HMRC, the arrests were part of a coordinated operation to tackle suspected abuse of the research and development (R&D) tax relief system.
This scheme is intended to support companies investing in innovative science and technology projects.
A spokesman said a number of other individuals had been invited to attend an interview under caution.
They would not confirm the names of the businesses raided, stating: “We do not comment on identifiable taxpayers.”
One eyewitness in Norwich said they were stopped from entering the building by HMRC personnel and that officers were posted on each floor.
“I walked in in the morning when I was greeted on the staircase,” they said.
The witness added: “They asked me: ‘Who are you? Where do you work?’ and told me that I couldn’t go upstairs.”
“In the nicest way possible, they were like rats all over the building,” they said.
Another described how they saw at least ten officers who had arrived at 07.30 BST and remained at the premises all day.
Green Jellyfish and Kirby and Haslam are both located at the Union Building on Rose Lane in Norwich.
The name Green Jellyfish is used by a number of companies registered at the same premises and one, Green Jellyfish Ltd, was formerly known as “Kirby and Haslam 1” before registering a change of name in 2023.
Businessman Sotiris Christophi is listed as the person with significant control of Kirby and Haslam.
He could not be reached personally for comment.
‘We have nothing to hide’
Jonathan Smith, HMRC’s director responsible for agent compliance, said: “These arrests are just one small part of the comprehensive and wide-ranging action we’re taking to tackle suspected R&D fraud.
“We are committed to supporting honest businesses, and their agents, to get the tax reliefs they’re entitled to.
“We urge anyone with information about any type of tax fraud to report it to HMRC online.”
Companies can reduce their tax bill or claim payable cash credits as a proportion of their R&D expenditure.
A spokesman for Kirby and Haslam said: “We welcome the investigation from HMRC and understand they have to look into all claims made.
“We have been and will continue to be fully cooperative as we have nothing to hide,” they added.
Green Jellyfish said in a statement: “We understand that HMRC has a job to do, and we are fully cooperating and supporting them with the investigation, as we have nothing to hide”.