Hammarberg told RFE/RL's Armenian Service that he has not heard "any convincing arguments" that the seven planned to overthrow the government.
The trial of the seven activists began recently but has been postponed several times because they refuse to stand when the judge enters the courtroom.
They were arrested in March after opposition protests against what they called "fraudulent elections" turned violent.
There are some 60 opposition activists jailed in Armenia.
The Council of Europe's Parliamentary Assembly (PACE) decided on January 27 not to suspend the Armenian delegation's voting rights in the parliament because of Yerevan's pledge to change two articles of its criminal code that Hammarberg said "invite human rights violations."