European stocks extended their early losses on Tuesday, suffering their biggest one-day sell-off in six weeks after Greek prime minister called a referendum on the latest bailout deal, risking a new euro zone crisis.

At 8:31 a.m., the FTSEurofirst 300 <.FTEU3> index of top European shares was down 2.8 percent at 968.30 points, with Societe Generale down 12 percent and Credit Agricole down 11 percent.

The Euro STOXX 50 volatility index <.V2TX>, Europe's main barometer of anxiety known as VSTOXX index, jumped 20 percent, hitting a more than one-week high.

(Reporting by Blaise Robinson)