Germany on Friday became the 23rd country in the world to legalise same-sex marriage, after lawmakers passed a bill in a snap vote that was approved by Chancellor Angela Merkel just four days ago.

The initiative, which delivers full rights to same-sex couples, including on adoption, was propelled through Germany's Bundestag (parliament) with the backing of 393 lawmakers to 226 against, reports Efe news.

"Progress is possible. As the 23rd country on earth, we now have marriage equality in Germany. I'm happy for all the married couples-to-be," wrote Martin Schulz on Twitter, whose Social Democratic Party (SPD) seized on Chancellor Angela Merkel's decision on Monday to allow the vote.

The SPD has several government ministers in office as part of its grand coalition agreement with Merkel's Christian Democratic Union, but broke ranks with the CDU in order to push for Friday's vote, sparking criticism from several conservative lawmakers.

Merkel voted against the measure, saying she personally believed marriage in German law is be between a man and a woman, although she thought same-sex adoption should be possible.

"I hope that the vote today not only promotes respect between different opinions but also brings more social cohesion and peace," she added.

The measure was backed by the SPD, the Left and the Green Party as well as members of the CDU, who were allowed a free vote.

Speaking during the debate ahead of the vote, SPD parliamentary chairman Thomas Oppermann argued that "if gay marriage is chosen, then many will receive something, but nobody will have something taken away".

In his debate intervention, CDU's parliamentary group leader Volker Kauder reiterated his ideological support for those who believe marriage should be available only for a man and woman, he added, however, he would respect those in his party who would vote in support.

Jan Marco Luczack, also from the CDU, defended legalising same-sex marriage, saying conservatives should defend the traditional values of love and harmony.

Prior to the vote, same-sex couples had access to civil unions.

Germany now joins 13 other European countries such as the Netherlands, Spain, the UK and Ireland, which already have same-sex marriage laws in place.