Rodrigo Duterte, the former president of the Philippines, was arrested on Tuesday in Manila, after the International Criminal Court issued a warrant accusing him of crimes against humanity in his war on drugs in which, human rights groups say, tens of thousands of Filipinos were summarily executed.

He was taken into custody at Manila’s main airport after returning from a trip to Hong Kong, according to the Philippine government. Mr. Duterte’s lawyer, Salvador Panelo, said the arrest was unlawful, partly because the Philippines withdrew from the court while Mr. Duterte was in office.

In the I.C.C. warrant, a three-judge panel wrote that, based on evidence presented by the court’s prosecutor, it believed that killings ordered by Mr. Duterte as mayor of the city of Davao and later as president were “both widespread and systematic.”