Scammers have created fanpages closely resembling the event's official page, advertising discounted entry fees, free race slots, recruitment opportunities and race bib transfers.
Victims are asked to transfer money for registration or race bibs before being added to private chat groups and encouraged to complete "tasks" to receive rewards.
They may then be persuaded to make purchases ranging from hundreds of thousands to hundreds of millions of VND. When victims try to withdraw their money, scammers demand additional payments for alleged errors, taxes or fees.
Quang Ninh's cybercrime police urged runners to verify information through official channels or contact organisers directly before registering.
People should only register through channels announced by the organisers and should not provide personal information, bank account details or OTP codes, or transfer money to unverified individuals.
Suspected scams should be reported to the nearest police station or Quang Ninh's cybercrime police.
The Halong Bay Heritage Marathon 2026 will take place from November 20-22, with the main races on November 22 at the Quang Ninh Planning, Fair and Exhibition Palace.
Organisers expect about 18,000 Vietnamese and international runners to compete in the 42.195-km marathon, 21.0975-km half marathon, 10-km and 5-km events.
The race is the only event in Vietnam currently holding a World Athletics Label Road Race certification, according to the organisers.