A Ukrainian lawmaker was detained by Russian police on March 1 ahead of a march in Moscow in memory of slain Kremlin critic Boris Nemtsov.
Oleksiy Honcharenko was later released from police custody but must appear before a judge on March 2 and could still face detention, his lawyer was quoted by the French news agency AFP as saying.
Honcharenko told RFE/RL that there was no reason for his arrest.
"I did nothing," he said, speaking from the Ukrainian Embassy in Moscow. "I wore a T-shirt with a portrait of Boris Nemtsov and the Ukrainian words 'Heroes Don't Die."
He said that he was beaten and taken to jail, where he spent more than five hours. He said he was refused a lawyer and a request for medical treatment was ignored. He said his arrest was a "violation of all laws of Russia."
The federal Investigative Committee said Honcharenko was being questioned about his alleged involvement in a fire last year that broke out in his home city of Odesa during rival demonstrations by Ukrainians and pro-Russian separatists. Dozens died in the fire, including some Russian citizens.
But a lawyer for Honcharenko in Russia, Mark Feigin, said Honcharenko had refused to answer questions without lawyers or the Ukrainian consul being present.
"There was no questioning," Feigin said.
He said the hearing on March 2 is based on an accusation that Honcharenko refused to comply with police demands, an administrative offense that is punishable by 15 days in jail.
Ukrainian President Petro Poroshenko said on Facebook late on March 1 that Honcharenko was at the Ukrainian Embassy in Moscow after being released.
The speaker of Ukraine's parliament, Volodymyr Groisman, said the detention was a violation of international law because Honcharenko has diplomatic immunity.
The speaker urged Ukrainian Foreign Minister Pavlo Klimkin to take urgent measures to have the parliament member released.
"They told me that diplomatic immunity in Russia is a favor," Honcharenko said. "For them, it's nothing."
Another Ukrainian deputy, military pilot Nadia Savchenko, has been held in Russia since early July since being captured by pro-Russian separatists in late June.
She has been on hunger strike since December 13 to protest her detention on charges of involvement in the death of two Russian journalists who were killed in the conflict between government forces and Russian-backed separatists.
Savchenko also has immunity as a member of Ukraine's delegation to the Parliamentary Assembly of the Council of Europe, but Russian authorities have rejected Western calls for her release and said her fate will be decided in court.
Honcharenko told RFE/RL that he could not predict his future.
"All prognoses here are without any sense. I don't believe in Russian courts," he said.
He said he thought the decision over his charges was Russian President Vladimir Putin's alone.
With reporting by AFP and Interfax