A UK newspaper editor and his deputy were among four journalists arrested over alleged phone hacking.
It is understood that an internal email has been sent to Mirror Group Newspapers staff confirming that People editor James Scott and deputy editor Nick Buckley were held.
The other journalists arrested were former editor of the Sunday Mirror Tina Weaver and former deputy editor of the newspaper Mark Thomas.
Today's arrests brought the Mirror Group into the police investigation into alleged phone hacking for the first time.
Scotland Yard said three men, aged 40, 46 and 49, and a 47-year-old woman were arrested at 6am this morning on suspicion of conspiracy to intercept telephone communications.
The message said Mr Scott and Mr Buckley are being provided with legal support and that the newspaper group is co-operating with police.
Scotland Yard said the alleged conspiracy is being treated separately from the two plots being investigated at the now defunct News of the World, and its inquiry is focused on the Sunday Mirror in 2003 and 2004.
In a statement, the force said detectives working on Operation Weeting, its investigation into phone hacking, have "identified and are investigating a suspected conspiracy to intercept telephone voicemails at Mirror Group Newspapers".
It added: "This conspiracy is being treated as a separate conspiracy to the two being investigated at the now defunct News of the World newspaper.
"It is believed it mainly concerned the Sunday Mirror newspaper and at this stage the primary focus is on the years 2003 and 2004."
The suspects are being interviewed at various police stations in London while searches are being carried out at a number of addresses.
Officers said they plan to make contact with people they believe to have been victims of the suspected phone hacking in due course.
A Trinity Mirror spokesman said: "We can confirm that James Scott the Editor of the People and his Deputy, Nick Buckley, were arrested this morning as part of Operation Weeting. We understand that two former employees were also arrested this morning.
"The police are investigating allegations of phone hacking whilst they were on the Sunday Mirror during 2003 and 2004.
"We are co-operating with the police and we have no further comment to make at this stage."
In a statement to the Stock Exchange today, Trinity Mirror company secretary Paul Vickers said: "We note the arrests this morning of two current and two former journalists employed by the Group as a result of the Metropolitan Police Operation Weeting.
"The police are investigating allegations of phone hacking on the Sunday Mirror in the period 2003 and 2004.
"We take any allegation against employees very seriously and are co-operating with the police on this matter."
Four civil claims have been lodged against the Mirror Group over alleged phone hacking, which it is contesting.
In the company's results today, the firm said it has applied for two of the matters to be struck out entirely and has challenged the general basis for the other two.
Media lawyer Mark Lewis is dealing with phone-hacking claims for four high-profile individuals including former England football manager Sven-Goran Eriksson.
The three other claimants were Coronation Street actress Shobna Gulati, Abbie Gibson, former nanny for the Beckhams, and Garry Flitcroft, former captain of Blackburn Rovers.
Today Mr Lewis said: "I welcome the latest developments that the police are fully investigating hacking at another paper."
The four people arrested were bailed to return to various London police stations on dates in April.