In a statement today, the broadcaster said Usmonov has been denied regular and confidential access to his lawyer and had not been allowed any access to his family or colleagues.
Reports from the Tajikistan news agency Press.tj on 18 June accused the BBC correspondent of being a member of banned Islamist group Hizbut-Tahrir.
The BBC said today that the accusation "represents a breach of legal practice and a serious violation of presumption of innocence".
The statement goes on to say that the allegations are "unfounded and the BBC sees them as a serious threat to professional journalism and to freedom of expression in Tajikistan".
"Urunboy Usmonov, as a BBC journalist, is expected to cover all sides of any story and in the course of his work it is only natural that he would meet and interview people representing all shades of opinion.
"It is vital for all journalists to be able to report freely on any organisation without fear of harassment and intimidation."
According to the BBC, Usmonov suffers from a serious heart condition and has requested medical attention.
BBC demands release of journalists detained in Tajikistan
Guardian journalist beaten in Pakistan
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