The incident happened shortly after jury selection for the hush-money trial was completed
A prominent Russian nationalist writer, Zakhar Prilepin, was wounded in a car bombing on Saturday that Russia immediately blamed on Ukraine and the West.
TASS news agency quoted the interior ministry as saying one person had been killed in the blast in Nizny Novgorod region, about 400 km (250 miles) east of Moscow.
It separately quoted a source in the emergency services as saying the writer was wounded but conscious after the explosion.
Russian Foreign Ministry spokeswoman Maria Zakharova wrote on Telegram: "The fact has come true: Washington and Nato fed another international terrorist cell - the Kyiv regime."
She said it was the "direct responsibility of the US and Britain", but provided no evidence to support the accusation.
"We pray for Zakhar," she said.
TASS quoted Kremlin spokesman Dmitry Peskov as declining to comment in the absence of information from investigators.
Prilepin is a novelist who is known as an outspoken supporter of Russia's actions in Ukraine. Regional governor Gleb Nikitin said: "Law enforcement officers are now investigating the circumstances and causes of the incident. Zakhar is OK."
Two leading pro-war Russians have been killed in bombings since Russia invaded Ukraine in February last year.
Darya Dugina died in a car bombing near Moscow in August, while military blogger Vladlen Tatarsky was killed in a bomb attack in a St Petersburg cafe last month.
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