They said at their trial in Kuwait City on Saturday that they made the confessions after they were shown pictures of their badly tortured ringleader and interrogators threatened to harm members of their families.
Twenty-one men and one woman pleaded innocent when the trial opened last month.
They were also accused of killing policemen in an unprecedented series of deadly clashes this year in the small, oil-rich country.
The gang's alleged ringleader, Amer Al-Enezi, was captured in one of the clashes in January and died in hospital of what the Interior Ministry said was a heart attack.
"They took me down to see Amer Khalif [al-Enezi]," said Majed Mayyah al-Mutairi from the dock. "I swear to God they are not human, he was cut to pieces."
The 33-year-old defendant said he was not beaten but it "was enough" for him to see the tortured ringleader to sign a false confession.
Ten of the 37 defendants are at large and several more have yet to appear in court. The majority of those in custody face the death sentence or life in prison if convicted.
Hussam Youssef Abdul-Rahim, a Jordanian defendant, said state security threatened to sexually abuse his wife, who was detained in another room, if he didn't say he knew that members of the group had fought battles with police.
"I asked them to have mercy on me because I had undergone an operation on my right testicle, so they lashed me on it with a stick"
Hussam Youssef Abdul-Rahim,
"I asked them to have mercy on me because I had undergone an operation on my right testicle, so they lashed me on it with a stick," he told the criminal court.
Seven of the defendants had previously told the court that they confessed under duress, four of whom took off their shirts in the dock to display clearly visible scars on their backs.
Most of the defendants are charged with joining the Lions of Peninsula, a group "based on extremist ideology" and rebellious against state institutions.
Four policemen and eight suspected "terrorists" believed to be connected to the group were killed in clashes across the country in January.
The defendants include Kuwaitis - including the wife of the alleged ringleader, who is battling cancer - as well as stateless Arabs who have lived in Kuwait for decades without acquiring citizenship, a Saudi, a Somali and an Australian businessman of Arab origin.
The Australian, Tala Adri, complained to the court on Saturday that the defendants were detained three to a prison cell and allowed out only once a day to pray. The lights are kept on day and night, he and other defendants said.
Kuwait has been a US ally since
the 1991 Gulf war
One lawyer, Mubarak al-Muttawah, declared the trial unconstitutional because the accused were not able to meet with their lawyers before it started.
Kuwait has been a US ally since the 1991 Gulf War when the Americans drove the invading Iraqis out of the country after their seven-month occupation.
However, groups who oppose the American military presence in Kuwait have attacked Americans since 2002, killing one US Marine and a military contractor.
Kuwait's Islamic movements condemned the January clashes and offered to help the government fight "terrorism".