Andrey Bandarenka of the United Civic Party was found guilty of financial mismanagement and sentenced to seven years in jail last month. He has been on a hunger strike since October 19 in protest of the sentence.
Bandarenka and his supporters say the charges against him were politically motivated.
Bandarenka's lawyer, Zmitr Laeuski, told RFE/RL that his client looked very sick when he saw him on November 4.
Laeuski said he is trying to arrange a meeting with Bandarenka to get a clear picture of his current condition.
Bandarenka was a candidate in the Belarusian parliamentary elections last year but was not elected.
Minsk authorities filed a lawsuit against him after he tried to initiate a check of the election results after he charged that they might have been rigged to keep him from winning.