BERLIN (AFP).- Pro-refugee activists in Germany have stolen seven memorial crosses for people killed at the Berlin Wall and moved them to spots on the EU's borders, they and police said Monday.

The action was meant to protest the "murderous exterior walls" of the EU which had claimed many thousands of lives, said a spokesman for the activist art collective "Centre for Political Beauty".

The white, cross-shaped signs bore the names and death dates of people killed while trying to flee communist East Germany, a regime which collapsed after the fall of the Berlin Wall 25 years ago next Sunday.

The seven signs were unscrewed from their frames set in a metal fence on the south bank of Berlin's Spree River near the site of the Reichstag building housing the lower house of parliament.

"We have moved the crosses to safety," said the group's Philipp Ruch, adding that some had been taken to EU borders in Bulgaria and others to the Spanish exclave of Melilla in North Africa.

Ruch also said the group and supporters would on Friday travel by bus to a secret place along the EU frontier with a plan "to dismantle the borders with bolt cutters".

On their website the group said the action would coincide with weekend celebrations marking the collapse of the Iron Curtain in Berlin a quarter-century ago.

"While in Berlin balloons will rise into the air and nostalgic, sedative speeches will be held, German civil society will in an act of political beauty bring down the European external walls," the group said in an online statement, adding that "the European Curtain must fall".

Berlin police said they were investigating the disappearance of the cross-shaped memorial plaques as "a particularly severe case of theft" with "a political motivation".

Rights group Amnesty International says the EU spent two billion euros ($2.5 billion) between 2007 and 2013 to fortify its outside borders against illegal immigrants, compared to 700 million euros to help asylum seekers.

According to the UN High Commissioner for Refugees more than 2,500 people have drowned or gone missing this year while trying to cross the Mediterranean from Africa to Europe.

On Monday, at least 21 people drowned and a dozen were missing when an overloaded boat taking migrants towards EU waters sank in the Black Sea just off Istanbul, Turkish officials said.