OAKLAND, Calif. (AP) _ Dart Group Corp., which is attempting to acquire Safeway Stores Inc. for nearly $4 billion, says the country's largest supermarket chain has received a bid from an undisclosed third party.

Dart, a Landover, Md.-based retail concern, said in a letter to Safeway's board of directors that the Oakland company was violating federal securities law by taking ''colusive action'' with the third party.

There was no immediate response from Safeway, which is based in Oakland. But the supermarket giant has been exploring ways of thwarting Dart's takeover.

Dart said if Safeway's board sells the company to a third party it would sue the directors.

''The litigation will escalate enormously,'' said Dart attorney Robert Hirsch. ''We will hold each of the board members accountable for their actions.''

''It is clear that you are savoring another bidder at any cost,'' Dart's chairman, Herbert Haft, and his son, Robert, who is Dart president, said in the letter, which was disclosed Thursday.

Dart has been stalking Safeway for three months and on Monday raised its bid to $64 a share, or $3.9 billion.

Hirsch said a Safeway official notified Dart that Safeway directors would meet with a ''third party'' whom they refused to name.

''It seems the third party told Safeway that if they told us who they were or what their price was, they wouldn't bid,'' said the attorney. ''We are outraged.''