SYDNEY, Australia — The Australian authorities tightened security at airports across the country on Sunday after disrupting what they described as an elaborate plan to detonate a bomb on a plane.

“We believe it’s Islamic-inspired terrorism,” the federal police commissioner, Andrew Colvin, said at a news conference. “Exactly what is behind this is something we need to investigate fully.”

Officials offered few details about the alleged plot, which they described as credible and organized. The police said four men had been arrested in raids at five locations in the Sydney suburbs on Saturday night. It was not clear how or if they knew each other, or whether they were linked to international terrorist groups.

Justice Minister Michael Keenan said the plot was the 13th significant threat broken up by the police since Australia raised its terrorist threat level from medium to high in 2014.