JAKARTA, Indonesia — Tunggal Pawestri says she’ll never forget being groped on a public bus while traveling to her high school in Jakarta, the Indonesian capital, when she was 14.

While Ms. Tunggal had become used to enduring daily harassment on her way to and from classes — mostly catcalls and sexually suggestive looks and comments — when a man suddenly began gyrating against her from behind, she said, “I froze.”

“I didn’t know what to do — I didn’t even know that I should have screamed,” said Ms. Tunggal, who now works for a women’s organization.

Two decades after that disturbing episode, a growing number of activist groups and volunteers like Ms. Tunggal are emerging to explain exactly what to do: expose the longstanding problem of harassment on roads, sidewalks, trains and buses across Indonesia, the world’s fourth-most populous nation.