A ship whose toxic waste caused the deaths of eight people and the collapse of the Government in Ivory Coast has been impounded in Estonia.

Estonian authorities in the Baltic port of Paldiski said today that a criminal investigation had been opened to explore the recent actions of the Probo Koala, a Panamanian-registered, Greek-owned oil tanker which discharged more than 500 tonnes of waste in the Ivorian capital of Abidjan in late August.

The waste — described by the oil trading company that chartered the ship as "Basle slops" or the residue from oil tanks that had been recently been cleaned — killed a reported eight people, including four children in Abidjan.

A stinking cloud of fumes hung over the city and around 9,000 people complained of rashes, vomiting, diarrheoa, migraines and nosebleeds.

The ill health and rumoured corruption surrounding the dumping of the waste led to angry street protests in Abidjan during which the country's Transport Minister was pulled from his car and beaten.

The house of the director of Abidjan was burnt down. On September 6, most of the Ivorian Cabinet resigned, prompting President Laurent Gbagbo to ask his Prime Minister, Charles Konan Banny, to form a new government.

The Estonian state prosecutor said today that the ship would not leave Paldiski until investigations were complete: "We have impounded the ship as it is under criminal investigation," Piret Seeman told Reuters.

The authorities acted after a request from the Ivorian Government to stop the ship. Preliminary tests showed that the vessel "contains similar substances as those in the Ivory Coast", said a spokesman for the Estonian Environment Ministry.

Environmental activists have also been following the ship and last night protesters hooked themselves up to the Probo Koala's mooring lines to stop it leaving. Greenpeace said the activists had moved in when they heard the ship, which they have tried to blockade by anchoring a protest vessel nearby, was about to sail.

Concerns about the waste on board the Probo Koala were first raised by port workers in Amsterdam in July. According to an investigation by the German magazine, Spiegel, a Dutch waste disposal operator, Amsterdam Port Services (APS) refused to accept the ship's waste because of high concentrations of a substance known as mercaptan, a poisonous residue of decaying crude oil.

Rather than pay to dump the waste at a special facility, and wary of a $250,000 (£131,000) penalty for arriving late at its next port of call, the Probo Koala left Amsterdam with the waste on board.

The company chartering the ship, a Dutch oil trading firm called Trafigura, which was fined $17.9 million (£9.4 million) for its role in the UN's Oil-for-Food scandal, ordered it to the African coast.

Environmental activists are sceptical about the circumstances of the deal that eventually allowed the ship to get rid of the waste.

A small company, called Tommy, and wholly owned by the Puma Energy Group, in which the Ivorian President's family is reported to own shares, won the contract to accept the vessel's slops.

In late August, tonnes of waste, which Trafigura insists was of low toxicity, was dumped at night at more than 11 sites across Abidjan, leading to the health crisis.

Last week, Trafigura's company president, Claude Dauphin, was arrested in Ivory Coast on what the company called "a high level humanitarian visit". He is currently in custody in Abidjan with the company's West Africa manager, Jean-Pierre Valentini.

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