TPLF arrests OFDM general secretary obbo Bekele Jirata.
"He was working hand-in-glove with terrorists," Bereket Simon, special advisor to Meles, told Reuters.
"It is proven he had links with groups like the OLF." Said Berket to Reuters.
Opposition groups say they are constantly harassed by the government despite operating within the law.
"At this time, we are unable to have any meaningful dialogue with the authorities who ordered the detention," OFDM chairman Bulcha Demeksa told Reuters in an e-mail.
"The constitution and other human rights laws would have been our guarantor, but...this is not the case in Ethiopia."
The OFDM in April accused the government of intimidation as voters went to the polls for the first time since post-election violence in 2005 killed 199 civilians.
The party said most of their candidates in the April local elections had been threatened and forced to pull out of the race. They ran less than two percent of their originally proposed candidates.