German prostitutes are campaigning to ensure any future government does not withdraw their workers' rights, as some MPs have proposed, the BBC's Ray Furlong in Berlin reports.
Some prostitutes are failing to take advantage of new rights
It was an unconventional rendition of the German national anthem, going intermittently out of tune as the singers' voices didn't quite make the high notes.
But this was no ordinary choir or concert.
A group of about a dozen women had gathered in front of the Reichstag to deliver a lyrical warning against repealing the prostitution law which Germany approved in 2002.
The protest was organised by the prostitutes' advice project Hydra.
According to its rewritten version of the national anthem, victory for the opposition Christian Democrats (CDU) in a general election expected in September will return prostitutes in Germany "to the dark ages".
"These politicians want to change the law that exists since 2002," said Hydra representative Katharin Cetin.
"This law has the advantage that prostitutes have the opportunity to have unemployment insurance and health insurance. So, if they cut this the prostitutes won't have these rights again."
But inside the Reichstag, there was little sympathy for the protest.
Christian Democrat MP Ute Granold argued that legalising prostitution three years ago was a mistake that needs correcting.
"Prostitution is not a job like any other one," she said. "It is a violation of human dignity.
"The women are either forced to do it, or they need money. The law wasn't good. Nobody, no woman, has gone to insure herself. They don't want it. Therefore, we will make a new law."
But criticism of Germany's prostitution law comes not only from conservative politicians.
Many of its supporters also feel it has failed to meet the high expectations that it created.
"It was only a small, small step of what we expected, of what we need, and what is very important is to give us a legal, fair position in our work," says Stephanie Klee, a prostitute for 20 years and a lobbyist for sex workers' rights.
She says many sex workers failed to take advantage of the new rights provided for them by the new law because it failed to remove the social stigma that still surrounds their job.
"For the family and the friends, and the neighbours, the woman tries to explain that she's working in another field. Sometimes she also tells her husband that she's working in an office, or that she's working in a restaurant, and this makes it very complicated for her.
"But when she is open with it, her husband, and also children, may be discriminated against."
Germany's main service industry union, Verdi, has tried to persuade prostitutes' groups that sex workers need to join the union if they want a better deal.
Last month, it presented a model contract between prostitutes and brothel owners. But Verdi's efforts have met with scepticism from sex workers themselves.
"There are still legal problems - such as laws that contradict the prostitution law, meaning prostitutes don't have an equal status with other professions," says Emilija Mitrovic, who carried out a study for the union.
"Often, when sex workers try to register with tax offices they are charged back tax for the previous five or 10 years. There are also many federal states which say they won't implement the new law. So they keep registering prostitutes, something they don't do with other professions."
Another problem with the prostitution law is that it offers no protection to foreigners without work permits, who make up almost half of Germany's 400,000 prostitutes.
Despite the progress it brought, most sex workers here still have very poor conditions and the prospects for improvement are unclear.