The mayor of Amsterdam has written to his counterparts in eight other European capitals, calling on them to uphold gay rights.
Gay rights demonstrators defied a ban in Warsaw last year
In his letter, Job Cohen warns that intolerance against homosexuals is on the increase.
The Netherlands introduced gay marriage five years ago, and Mr Cohen is urging other countries to do the same.
Mayors in Warsaw, Prague, Lisbon, Dublin and Vienna are among those who have been sent copies of the letter.
Mr Cohen has also written to leaders in the Baltic capitals of Riga, Tallinn and Vilnius, and EU Justice Commissioner Franco Frattini.
The move is a result of a motion by city councillors who are concerned over recent attacks on homosexuals in Amsterdam.
In the letter, Mr Cohen says he is "concerned" by measures in some cities to ban gay demonstrations.
He calls on mayors to "adhere to the universal declaration of human rights, and to do everything in your political power to open up marriage for same-sex couples and safeguard the right of public demonstrations in your city," Agence France-Presse news agency reports.
In February, Portuguese officials turned down an application by a lesbian couple to get married.
Official statistics show that 8,000 same-sex marriages have taken place in the Netherlands since they became legal, AFP said.