TOKYO — Tens of thousands of people on the Japanese island of Okinawa gathered on Sunday to demand the removal of American military bases in what organizers said was the largest demonstration against the United States presence there in two decades.

The protest, in Naha, the capital of Okinawa Prefecture, was billed as a memorial for a 20-year-old woman who was found dead last month. A United States Marine veteran who was working as a civilian contractor on the island has been arrested in connection with the killing, prompting a public outcry.

Organizers said 65,000 people had attended the protest. That would make it the largest demonstration since 1995, when two American Marines and a Navy sailor were arrested over the rape of a 12-year-old girl, an episode that shook the tight military alliance between the United States and Japan and is still bitterly remembered by many Okinawans.