SAN FRANCISCO — Just days after Amgen announced its $10.4 billion purchase of Onyx Pharmaceuticals, shareholders sued to stop the acquisition, insisting the deal benefits insiders at the expense of ordinary investors.

Plaintiffs lawyers at Robbins Geller Rudman & Dowd and Milberg filed suits this past week seeking to block Amgen from taking over the South San Francisco-based biopharmaceutical company. If approved, the deal would be one of the year's largest healthcare mergers.