The editor and deputy editor of the People, and two former Mirror Group journalists, are arrested by police investigating alleged phone hacking.
It is the first time Mirror Group Newspapers has been dragged into the police hacking investigation.
Those arrested are People editor James Scott, his deputy Nick Buckley, former Sunday Mirror editor Tina Weaver and her former deputy Mark Thomas.
They were arrested on suspicion of conspiracy to intercept telephone communications.
An internal email sent to Mirror Group staff said Messrs Scott and Buckley were being provided with legal support and the group was co-operating with police.
Scotland Yard said the alleged conspiracy was being treated separately from the two plots being investigated at the now defunct News of the World. The inquiry is focused on the Sunday Mirror in 2003 and 2004.
The Metropolitan police said in a statement that detectives working on Operation Weeting, its investigation into phone hacking, had “identified and are investigating a suspected conspiracy to intercept telephone voicemails at Mirror Group Newspapers”.
The suspects are being interviewed at various police stations in London and searches are being carried out at a number of addresses.
Officers 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.”
Four civil claims have been lodged against Mirror Group over alleged phone hacking, which it is contesting.
In the company’s results today, the firm said it had applied for two of them to be struck out entirely and had 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 are 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.”