The Netherlands introduced gay marriage five years ago, and Mr Cohen is urging other countries to do the same.

Mr Cohen's letter was sent to mayors in Warsaw, Prague, Lisbon, Dublin, Vienna and in the Baltic capitals of Riga, Tallinn and Vilnius, as well as to EU justice commissioner Franco Frattini.

The move comes after a motion by city councillors, concerned by recent attacks on homosexuals in Europe.

In the letter, Mr Cohen says he is alarmed by measures in some cities to ban gay demonstrations.

He calls on his colleagues 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."

When he was mayor, Polish president Lech Kaczynski tried to block a gay pride march in Warsaw in June while Portugal recently turned down a lesbian couple's request for a marriage licence.

Recently the Latvian and Lithuanian parliaments decided to table amendments to their constitutions prohibiting same-sex marriage.

But countries such as the Netherlands, Belgium, Spain and the UK have recognised same-sex marriages or partnerships.

On 1 April 2001, Mr Cohen became the country's first public official to wed same-sex couples. The Netherlands was one of the first country in the world to allow gay and lesbians to legally marry.

Official statistics show that 8,000 same-sex marriages have taken place in the Netherlands since they became legal.