Libya PM denies Gaddafi premier in "critical condition"

* Ex-PM first senior Gaddafi official returned for trial

* Lawyer said Mahmoudi tortured; dismissed by current PM

* Rights groups worried about trials inside Libya

TRIPOLI, Feb 28 (Reuters) - Libya's prime minister dismissed on Thursday reports that Muammar Gaddafi's former premier was in a critical condition after bring tortured in prison.

Al Baghdadi al-Mahmoudi was extradited from Tunisia in June, making him the first senior Gaddafi official to be returned for trial under Libya's new leadership.

He went on trial in November charged with corruption and ordering mass rape during the 2011 conflict that toppled Gaddafi and is being held in a Tripoli prison.

On Tuesday, his Tunisian lawyer, Mabrouk Khorchi, said Mahmoudi was in critical condition and "risks dying" after being tortured in jail.

"Al-Mahmoudi is in good health, and is being treated humanely," Libya's present Prime Minister Ali Zeidan told a news conference.

"Our duty is to preserve his dignity ... The Mahmoudi trial is in the hands of the Libyan judicial system. The court will decide whether he is guilty or innocent."

An officer at the prison where Mahmoudi is being detained also denied he was in bad health.

Mahmoudi served as Gaddafi's prime minister from 2006 until he fled to neighbouring Tunisia in August 2011 around the time rebel fighters took Tripoli. He could face life in jail or execution if convicted.

Human rights groups have questioned whether former officials can get a fair trial in a country where bitterness over Gaddafi's rule runs deep and the militias that helped unseat him have influence.

But Libya's new leaders have said they are determined to show their state institutions are up to the job of conducting such trials. (Reporting by Ali Shuaib; Writing by Marie-Louise Gumuchian; Editing by Andrew Heavens)