Senussi was extradited from Mauritania to Libya in early September. He, along with Moammar Gadhafi's son Saif al-Islam Gadhafi, is wanted by the International Criminal Court on charges of committing crimes against humanity during civil war in 2011.
Both men are in custody in Libya, which maintains the right to try them in national courts.
Senussi told Human Rights Watch this week he hasn't spoken to a lawyer nor has he been informed about the charges he may face.
Director of Middle East programs at Human Rights Watch Sarah Leah Whitson said Libya has a right to try former regime officials in national courts.
"But to achieve true justice, they need to give Senussi the rights that the previous government denied Libyans for so long," she said in a statement from Tripoli. "To start, that means making sure he can consult a lawyer."
The rights group said Senussi faces Libyan charges of playing a role in the massacre of more than 1,000 prisoners in 1996.
He told the rights group he didn't know of any representation available to him at The Hague, but was ready to face the charges against him.
He said he wasn't being mistreated, describing his detention conditions as "reasonable."
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