A corporate fight continued Wednesday over California-based Longs Drug Stores Corp., one of the last major regional drugstore chains, when it rejected a $2.8 billion offer by Walgreen Co.

Last week Walgreen, the largest retail pharmacy chain in the country, made an eleventh-hour bid to scuttle the previously announced agreement in which CVS Caremark Corp., the second-largest chain, would pay about $2.7 billion for Longs.

In declining the offer, Longs, based in Walnut Creek, said it had concerns about potential antitrust risks and noted that Walgreen hadn’t outlined a “clear road map” to acquire the company.

But Walgreen, based in Deerfield, Ill., said the battle was far from over.

“We are disappointed with the refusal of the Longs board to discuss our superior proposal,” spokesman Michael Polzin said. “Longs stockholders and the marketplace as a whole have demonstrated strong support for a transparent and fair process that more fully values the company. We remain committed to pursuing our proposal.”

Longs operates 521 stores in four states, mostly in California but also in Hawaii, Nevada and Arizona. The chain also provides pharmacy benefit management services through its RxAmerica subsidiary.