Armed Syrian opposition groups have been accused of "serious human rights abuses" by an international watchdog, even as government forces continue to commit atrocities against civilians.
But in a public letter to the Syrian National Council (SNC), which is based in Turkey, it made clear that opposition fighters have also carried out kidnappings, torture and executions against government supporters and militia members.
HRW called on supporters to condemn and forbid members from carrying out the attacks.
"The Syrian government's brutal tactics cannot justify abuses by armed opposition groups. Opposition leaders should make it clear to their followers that they must not torture, kidnap, or execute under any circumstances.
Sarah Leah Whitson, Middle East director at Human Rights Watch, said:
"The Syrian government's brutal tactics cannot justify abuses by armed opposition groups.
"Opposition leaders should make it clear to their followers that they must not torture, kidnap, or execute under any circumstances."
The Free Syrian Army (FSA) has taken an increasingly prominent role in the uprising in the last few weeks after its peaceful beginnings broke down into violence.
But the FSA is not a unified fighting force, and is made up of many disconnected groups.
"The Free Syrian Army is more a brand than a meaningful, unified force," wrote the the Saban Center at Brookings in a recent memo. "Al- Qa'ida is urging fighters to join the fray in Syria, and sectarian killings and atrocities are growing."
Witnesses told the HRW that they had been beaten with electrical cables by opposition figures.
One, 'Mazen', told HRW he was captured by fighters in Saraqeb: "The detainee told me he was a first assistant in the National Hospital in Aleppo.
"I asked the revolutionaries to bring him to me so I could speak to him. He said that opposition fighters had beaten him with electrical cables on the head ... and he was blindfolded. He said they let him speak to his parents ... and that they asked them for money to release him."
The HRW also said it was concerned that the FSA had kidnapped Iranian civilians because it was suspected they were working with President Bashar al-Assad.
Increased fighting between the Syrian army and opposition fighters has been reported in recent days.
Witnesses have reported rocket attacks and machine gun fire in the capital, and several bombings were reported by Syrian state TV which blamed the blasts on "terrorists".
Russia has since joined the International Committee of the Red Crossin calling for a daily two-hour ceasefire to help humanitarian aid reach wounded fighters and civilians.
Previously Russia had vetoed Western-led resolutions in the United Nations Security Council because as well as calling for aid they also condemned Assad and called for his removal from power.