By Basil Katz

NEW YORK (Reuters) - A U.S. judge refused on Friday to overturn the conviction of Ahmed Khalfan Ghailani or grant him a new trial, clearing the way for sentencing of the first Guantanamo detainee to face a civilian trial.

A U.S. jury in November convicted Ghailani on one count of conspiracy to damage or destroy U.S. property with explosives relating to the 1998 bombings of U.S. embassies in Africa, but cleared him of 284 other conspiracy and murder charges.

Ghailani, a 36-year-old Tanzanian currently being detained in New York City, is due to be sentenced on January 25. He faces a minimum of 20 years in prison.

His five-week trial was seen as a test of U.S. President Barack Obama's approach to handling the 173 terrorism suspects now held at the U.S. naval base at Guantanamo Bay, Cuba, including Khalid Sheikh Mohammed, the self-professed mastermind of the September 11, 2001 attacks on the United States.

Ghailani was accused of participating in the embassy bombings in Kenya and Tanzania that killed 224 people. His lawyers argued that he had run errands for men he realized were al Qaeda operatives only after the attacks.

U.S. District Court Judge Lewis Kaplan, however, ruled that the government had established sufficient proof to support the conviction.

"The evidence of Ghailani's culpable mental state and intent was plentiful," Kaplan wrote in an opinion unsealed on Friday, rejecting defense requests that Ghailani be cleared of all charges or retried on the one conspiracy charge.

Kaplan speculated the verdict that followed the trial in Manhattan may have been "a bargain in the end" with 11 jurors in favor of conviction and one against "so everyone could go home."

Defense attorneys admitted that Ghailani bought a truck and gas tanks used in the car-bombings without knowing how they would be used. FBI officials testified they found traces of explosives on his clothes and a blasting cap in his armoire.

A spokeswoman for the Manhattan U.S. Attorney's office declined to comment on Kaplan's ruling. Lead defense attorney Peter Quijano also declined to comment.

Obama vowed during his 2008 presidential campaign to close the controversial Guantanamo prison, a pledge that has run into resistance from critics who argue that it is needed in the battle against Islamic extremists.

(Reporting by Basil Katz; Editing by Daniel Trotta and Paul Simao)