Some of the Afghans said upon arrival in Kabul on Tuesday they had been mistreated by their American prison guards.
Abd al-Rahman, who appeared to be in his mid-30s and was among three men allowed to speak to reporters after being handed over to Afghan authorities, said: "They used extreme type of tyranny against us."
Some said they had been in detention since the 2001 fall of Afghanistan's former Taliban rulers in a US-led invasion.
The 18 who were released in the largest single exodus from the US naval base at Guantanamo Bay, Cuba, since last September were among 38 detainees who the Pentagon had decided no longer were considered enemy combatants.
Maj Michael Shavers, a Pentagon spokesman, said another 15 detainees who also no longer were classified as enemy combatants remained at Guantanamo awaiting transfer to their home countries. Five others previously were released.
Legal black hole
The US still holds approximately 520 prisoners at Guantanamo after freeing 167 to their home countries and sending 65 more to their home governments for continued detention, the Pentagon said. Many detainees have been held for more than three years.
Human-rights activists have accused the US of condemning Guantanamo prisoners to indefinite detention in a "legal black hole," and note that some former detainees have said they were tortured by US personnel at the base.
"They used extreme type of tyranny against us"
The US has classified prisoners sent to Guantanamo as enemy combatants not entitled to the rights accorded to prisoners of war under the Geneva Conventions.
Following a June 2004 Supreme Court ruling allowing Guantanamo prisoners to go to US courts to challenge their detention, the Pentagon created special three-member military panels to review whether the detainees had been properly classified as enemy combatants.
These panels which conducted hearings for every Guantanamo prisoner, ended on 22 January. Some 38 were found not to have been enemy combatants, the Pentagon said in March. The prisoners were prohibited from having lawyers in this process.