Former Tajik Prime Minister Abdumalik Abdullojonov has been detained at Boryspil Airport southeast of Kyiv on an international arrest warrant.
Ihor Ischuk, a press official at the Ukrainian Interior Ministry, confirmed to RFE/RL's Ukrainian Service that an arrest was made on February 5.
"Representatives of the General Directorate for Combating Organized Crime detained a -year-old man, who for 16 years has been wanted by the Interior Ministry of Tajikistan," Ischuk said.
"The man has been wanted for a number of very serious crimes. Until now, he has been hiding in the United States. He came to Ukraine for his own business and did not expect at all to be detained. Now, the question of his extradition is being considered."
Abdullojonov's detention was also confirmed to RFE/RL's Tajik Service by an official with the international police agency, Interpol, in Ukraine.
Abdullojonov, 64, is accused by Tajik authorities of involvement in a 1996 assassination attempt on Tajik President Emomali Rahmon.
He is also accused of backing a 1998 militant attack in Sughd Province and of organized crime and terrorism-related offenses, as well as a number of economic crimes.
Abdullojonov rejects the charges as politically motivated.
Although he left Tajikistan in the 1990s, Abdullojonov is still referred to by media outside the country as a serious political rival of the Tajik president.
He challenged Rahmon in the 1994 presidential election in which Rahmon was declared the winner. Abdullojonov claimed to have won and accused Rahmon of electoral fraud.
Saying he feared prosecution, Abdullojonov left for Russia. Since 1998, he has lived in the United States.
Abdullojov's arrest comes amid expectiations that Rahmon will run for another term in the next presidential election scheduled for November.
A number of Rahmon's political opponents have been arrested in other countries at Tajikistan's request over the past decade.
The former leader of Tajikistan's Democratic Party, Mahmadruzi Iskandarov, was arrested in Russia and extradited to Dushanbe in 2005. Iskandarov is serving a 23-year prison term after being found guilty on a series of charges ranging from terrorism to economic crimes.
Former Interior Ministry Yoqub Salimov, who was also arrested in Russia at the request of Tajik authorities, was sentenced to 15 years in prison in 2005. He was found guilty of plotting to overthrow Rahmon's government.
In December, the relatively unknown leader of the opposition movement Group24 was arrested in Dubai. Tajik authorities are seeking Umarali Quvatov's extradition from the United Arab Emirates.
Tajik authorities accuse him of committing fraud of some $1.2 million. Quvatov, a successful businessman, used to have close ties to Rahmon's family.