Twenty people were arrested when Greenpeace protesters boarded a single-hull tanker in the bay of Algeciras off Gibraltar, a police spokesman said.
Local police said they included six Spanish journalists, although Greenpeace's director for Spain, Mr Juan Lopez de Uralde, said there were at least 14 journalists among those detained.
Mr Lopez de Uralde said the protest aimed to call attention to the European Union's failure to crack down on the transport of hazardous substances since the single-hulled Prestigesank off Spain disaster.
The Greenpeace vessel staged the protest early today, offloading two dinghies from which protestors boarded a tanker and climbed up its mast.
A Gibraltar police launch in the area was also rammed by a Greenpeace dinghy, according to police who said the 20 were detained for obstructing police and for dangerous navigation. It was not clear if they would be charged.
The Prestigehas dumped tens of thousands of tonnes of heavy fuel oil since it sank in November and caused the worst pollution every on Spain's coast.
Algeciras bay, between Gibraltar and the Spanish port of the same name, "is a time-bomb and Greenpeace demands an urgent and total ban on single-hull tankers and a new regime of unlimited liability," the Greenpeace director said.
The European Commission has called on EU members to ban single-hull tankers carrying heavy fuel oil from their ports by March 2003 and proposed a gradual phase-out, between 2005 and 2010, of single-hull tankers carrying other hydrocarbons.
AFP