Government officials appealed for calm on Friday after three days of attacks by ultranationalist mobs against dark-skinned foreigners in Athens, set off by a fatal mugging. More than 100 Asian and African immigrants were attacked by rampaging youths on Thursday, a Pakistani community spokesman said, and dozens of immigrant-owned shops were attacked or looted. Tens of thousands of illegal migrants from Asia and Africa have moved into depressed neighborhoods in the center of Athens, fueling a nationalist backlash. Nobody has been arrested for the killing on Tuesday; some have blamed immigrants. The public order minister, Christos Papoutsis, said there was a “very high risk of hate crimes” and promised action to address inner-city crime.