KYIV -- Ukrainian authorities have arrested a key negotiator in prisoner exchanges with Russia-backed separatists and accused him of plotting to kill President Petro Poroshenko and other top officials.

A Kyiv judge laid out the accusations against Volodymyr Ruban at a hearing on March 9, less than 24 hours after he was detained while crossing into government-controlled territory in eastern Ukraine -- allegedly with large amounts of weapons and ammunition hidden in a shipment of furniture.

The judge said that investigators claimed Ruban planned to use mortars, grenade launchers, guns, and explosives to carry out "armed attacks on the residences of statesmen and political leaders," including Poroshenko, Interior Minister Arsen Avakov, former Prime Minister Arseniy Yatsenyuk, and National Security and Defense Council chief Oleksandr Turchynov "with the intention of killing them."

Ruban told the court he was innocent and suggested he had been set up, saying he was "not aware" that there were weapons hidden in the furniture. The judge ordered him held in pretrial custody for two months on suspicion of plotting terrorist acts and smuggling weapons.

Nadia Savchenko, a military aviator who was jailed in Russia for 22 months after being detained by separatists in the conflict zone in the east and is now a lawmaker, came to the courthouse to support Ruban.

After the hearing, Poroshenko tweeted that the Ukrainian Security Service (SBU) chief Vasyl Hrytsak would oversee the investigation.

Hrytsak later told reporters that Ruban was suspected of plotting a series of terrorist attacks in Kyiv and other parts of Ukraine in coordination with the leader of Russia-backed separatists in Ukraine's Donetsk region, Aleksandr Zakharchenko, and the separatists' tax chief, Aleksandr Timofeyev.

"We have managed to prevent wide-scale terrorist acts in Ukraine. Hundreds of thousands of civilians, as well as certain politicians, officials and lawmakers, could become victims of those acts," Hrytsak said, adding that "there is no doubt that the plot was ordered by curators from Moscow."

Heorhiy Tuka, Ukraine's deputy minister for temporarily occupied territories, alleged on Facebook earlier that Ruban had tried to smuggle a large amount of weapons and ammunition from the area held by Russia-backed separatists.

According to Tuka, Ruban pretended to be an ordinary citizen trying to move from the separatist-held area into government-controlled territory.

Prosecutor-General Yuriy Lutsenko said that Ruban was apprehended "not by chance," hinting that the SBU had been following his activities for some time.

Ruban's Center For The Release Of POWs has been involved in prisoner exchanges between Kyiv and Russia-backed separatists since 2014, when the conflict in eastern Ukraine erupted.

In the past, Ruban was involved in the activities of Ukrainian Choice, an organization that many in Ukraine consider pro-Kremlin.

The group is headed by Viktor Medvedchuk, who has ties to Russian President Vladimir Putin and has played a major behind-the-scenes role in exchanges of captives.

In February 2017, Ukraine's Border Service and the SBU accused Ruban of violating regulations for entering areas under the control of the Russia-backed separatists who hold parts of the Donetsk and Luhansk regions.

The war in eastern Ukraine has killed more than 10,300 people and displaced hundreds of thousands since April 2014, when it began after Russia fomented unrest following the ouster of Moscow-friendly President Viktor Yanukovych.

Yanukovych was driven from power by massive pro-European protests that erupted after he scrapped plans to sign a landmark deal with the European Union and tighten ties with Russia instead.