"This is the best moment in my career," he told the Premier League club's Web site."I am very, very happy and now I can concentrate just on playing football," said the 23-year-old. He joined Liverpool on loan from fellow Premier League club West Ham United in January 2007 before helping his new team reach the Champions League final in May. Liverpool spokesman Ian Cotton said: "Javier Mascherano came to Liverpool on a fixed term contract under an agreement disclosed to and approved by both the Football Association and Premier League.
"His contract has now been extended under the provisions of that agreement... We have no further comment to make," he added.
Our overriding thought is we are delighted to have signed a world class player on a long contract," chief executive Rick Parry said on the Web site.
Mascherano joined West Ham in a controversial double move with compatriot Carlos Tevez in August 2006 in which they were loaned to the London club by Media Sports Investments (MSI).
Liverpool gave no financial details on the deal, leaving undisclosed the size of any transfer fee paid to MSI.
British media reports in the January transfer window said Mascherano could cost about 17 million pounds ($33.7 million).
Argentina striker Tevez now plays for Manchester United.
Mascherano, a key member of the Argentina side voted FIFA's team of the year in 2007, began his career at River Plate before joining Brazilian club Corinthians in 2005.
A member of the Argentina side that reached the 2006 World Cup quarter-finals, Mascherano has 36 caps.