WARSAW—The popular mayor of a major Polish city died on Monday after being stabbed during a public address, sparking an uproar in a country torn by political tensions between the conservative government and the opposition.

Pawel Adamowicz, 53 years old, a prominent politician close to the opposition who was mayor of the Baltic port city of Gdansk, was speaking on stage during a televised charity event attended by hundreds of people on Sunday night when he was assaulted.