Fugitive anti-abortion activist James C. Kopp, wanted in an international manhunt for the alleged sniper murder of an Amherst, N.Y., doctor in 1998, was arrested Thursday at a village post office in western France, U.S. federal authorities said. The French police, acting upon information from the F.B.I., arrested Kopp, 46, after he picked up a package from New York that contained about $300 in cash, said officials. Kopp’s alleged victim, Dr. Barnett A. Slepian, long a subject of protests by anti-abortion forces, was shot to death by a high-powered rifle whose bullet came through his own kitchen window as he was talking to his wife and two children. The arrest of Kopp in France was said to have taken place without incident. The international search for Kopp had gathered momentum after the F.B.I. had tracked the wanted man to Ireland. The New York Times reports that he vanished from Ireland on March 12, apparently with a false passport. The F.B.I. reportedly picked up his trail late last week. Also arrested in New York, for allegedly helping Kopp remain on the lam, were Loretta Marra and Dennis Malvasi.