Rock star Chrissie Hynde was among 33,000 people who campaigned against the Spanish government to secure the release of the Greenpeace vessel Rainbow Warrior last week.
Greenpeace launched the campaign after the Spanish authorities impounded the vessel in June.
It was grounded when Greenpeace activists chained themselves to a nearby cargo ship carrying timber, which the group said had been illegally logged in Cameroon. The government then demanded a bond of EUR300,000 (£207,500) for its release.
Hynde, lead singer of rock group the Pretenders, joined Greenpeace supporters in calling for the release of the ship after she played a concert in Valencia.
"The Spanish government is more interested in persecuting the protesters, than the companies importing illegally logged wood," she said.
The crew used the opportunity to post a diary on the Greenpeace web site, noting the amount of illegal wood imported into Spain while the ship is impounded.
Supporters wrote to Spanish ministers and to the head of the Spanish merchant navy, requesting the ship's release, which was finally secured after 20 days.