Home Secretary John Reid has been accused of possibly putting efforts to deport foreign prisoners at risk after joining a series of dawn raids.
Mr Reid watched the raid on Tuesday morning
He watched a team of immigration officers raiding homes in London.
They were trying to arrest some of the foreign prisoners released without being considered for deportation.
Immigration experts and the Tories have raised fears the raids make it difficult for Mr Reid if he has to make decisions about deportations later.
Keith Best, head of the Immigration Advisory Service, said: "It could put him in an embarrassing situation if he was present at a particular arrest of someone over whom he then has to make a disinterested decision about whether they should be deported.
"The home secretary should stand above operational matters, to be a disinterested party making decisions in a judicial manner."
For the Conservatives, shadow home secretary David Davis echoed the fears, saying: "I do hope that Mr Reid took legal advice before he decided to undertake his photo opportunity this morning.
"It would be a dreadful outcome if in pursuit of some media coverage he jeopardised the process for anybody who ought to be deported."
Mr Reid's decision to take part in such a raid is highly unusual.
But a Home Office spokeswoman said: "The home secretary was there as an observer to see the dangers and difficulties of these operations at first hand and to understand the procedures.
"He played no part in the decision making."
The raids come after Mr Reid faced press criticism going to France for the Bank Holiday weekend with his wife, Carine Alder.
His officials stressed he had not been "on holiday" and had taken papers with him, as well as being in contact via telephone and email.
Since he was appointed as home secretary earlier this month, Mr Reid has had to deal with the release from jail of 1,019 foreign prisoners, including murderers, rapists and paedophiles, without consideration for deportation.
A Home Office spokesman said four arrests were made on Tuesday and all were taken to immigration deportation centres for processing.
"The visits to a number of domestic addresses in north London were conducted as part of Operation Scully, the ongoing Immigration and Nationality Directorate and police operation," he said.
"Mr Reid had earlier sat in on two early-morning briefings at separate immigration enforcement offices in London and issued a rallying call to officers who have been working round-the-clock to locate and detain the foreign prisoners.
"He then headed out with one of the teams shortly after 5am.
"Backed up by police officers from the Met, the home secretary witnessed immigration officers force entry to four properties."
The team arrested a Cameroonian national convicted of fraud offences and another "suspected ex-prisoner" also convicted of fraud, the spokesman said.
"The other immigration team involved in today's operation also made two arrests in the south east London area, one a Jamaican national who was being sought after being released from custody following a manslaughter conviction and an Angolan national who had been convicted of theft and deception," he added.
The spokesman said Mr Reid had thanked staff for their hard work and was pleased with the morning's proceedings.