Karina Moskalenko told Russia's Ekho Moskvy radio station the incident had prevented her from travelling to Moscow to take part in the trial of three suspected accomplices in the 2006 murder of journalist and Kremlin critic Anna Politkovskaya.

"People do not put mercury in your car to improve your health," Moskalenko, who spends much of her time in the French city of Strasbourg, told the radio station. "I am very concerned because there were children in that car."

Mercury is an element that occurs naturally but exposure to high levels can damage the brain, heart, kidneys, lungs, immune and nervous systems.

A high level of exposure in metallic form -- most commonly by breathing in vapour -- can lead to death, said the U.S government's Agency for Toxic Substances and Disease Registry.

"I think it may have been a demonstration because there was lots of it (mercury.) How could you not notice it?" She did not say who she thought might have been responsible.

Strasbourg assistant prosecutor Claude Palpacuer said an investigation had been opened. He said Moskalenko and members of her family had been invited to undergo a medical examination to check if they had been contaminated.

"The circumstances as well as her background are such that she is concerned," he told Reuters.

Moskalenko's clients have included Mikhail Khodorkovsky, founder of a major oil company who is serving a prison sentence for fraud and tax evasion. He said his prosecution was punishment for challenging the Kremlin's power.

Prosecutors applied to the Moscow Bar Association to have Moskalenko stripped of her right to practise as a lawyer, a step she said was designed to weaken Khodorkovsky's defence.

She has also represented several Russian plaintiffs who have appealed to the European Court of Human Rights in Strasbourg.

The court has handed down dozens of rulings accusing Russia of human rights abuses and failures of its justice system.

Anna Stavitskaya, a lawyer representing Politkovskaya's family, said her legal team would ask for the trial to be postponed because Moskalenko was unavailable. The trial is scheduled to start in a Moscow military court on Wednesday.

Alexander Litvinenko, an emigre Kremlin critic living in London, died in 2006 from radiation poisoning. British police have named a Russian former security agent as a suspect. He says he is innocent.