Phone hacking at the Daily Mirror, Sunday Mirror and Sunday People was "rife", the high court in London has heard.
Actress Sadie Frost and soccer legend Paul Gascoigne are among eight high-profile celebrities taking the Mirror Group Newspapers to court over hacking allegations between 1999 and 2006.
The others include TV stars Shane Richie, Shobna Gulati and Lucy Benjamin, producer Robert Ashworth and air hostess Lauren Alcorn who had a relationship with footballer Rio Ferdinand.
The court heard former Sunday Mirror journalist Dan Evans hacked the phones of celebrities at least 100 times a day between 2003 and 2004.
David Sherborne, the defence lawyer for the victims, said hacking at the titles was "utterly unprecedented".
"It is a reasonable inference that phone hacking was rife at all three of MGN's national titles at or around the same time, that is by mid-1999 at the latest," he said.
"This was not just the work of junior reporters ... quite the opposite.
"The evidence demonstrates that voicemail interception, as well as the unlawful obtaining of personal information by blagging or use of private investigators, was in widespread and habitual use by a large number of journalists across all three MGN titles."
Sherborne said journalists hacked the voicemail of different celebrities nearly 10,000 times.
The case is expected to last for two weeks.