TRIPOLI, Libya -- Abdullah el-Senussi, the former intelligence chief and brother-in-law of Col. Muammar el-Qaddafi, was arrested Saturday at an airport in Mauritania, the authorities there said, in the most significant capture of a former official of the Qaddafi government since the apprehension of the dictator's son Seif al-Islam by Libyan fighters in November. The International Criminal Court has indicted Mr. Senussi, Seif al-Islam and Colonel Qaddafi on accusations of crimes against humanity committed during the government's attempts to crush the rebellion last year that ultimately ousted the Libyan leader. Colonel Qaddafi was killed by Libyan fighters shortly after his capture in October. During his years as intelligence chief, Mr. Senussi presided over the surveillance, detention, torture and assassination of probably thousands of Libyans. Aside from Colonel Qaddafi himself, Mr. Senussi was the most reviled figure in the former government. [ABSTRACT FROM AUTHOR]