The 10-minute video shows an army officer casually questioning four men in a building. The officer then steps aside and soldiers move in, punching, kicking and whipping the suspects, who scream in pain and writhe on the ground.
The video has been posted on the Facebook and YouTube sites.
Army spokesman Maj. Gen. Athar Abbas said the army was investigating the alleged abuse, but declined further comment Friday.
Pakistan's poorly trained and underfunded security forces have long been accused of human rights abuses.
The independent Human Rights Commission of Pakistan said in an August report it had received "credible reports of numerous extrajudicial killings and reprisals carried out by security forces" in the Swat Valley since the area was retaken from Taliban control in July.
It was not clear where or when the video was shot, and its authenticity was impossible to verify. It was first posted on Facebook last month.
It shows the officer quizzing one man over whether his brother-in-law is a militant. The man says he does not know. The officer then signals for his deputies to begin attacking the suspect. He is punched, lashed with a leather rope and kicked repeatedly while on the ground.
He screams "Have mercy on me, oh God" in Pashto, the language of the northwestern tribal areas close to the Afghan border where the Pakistan army is engaged in anti-militant offensives. Two of the men who were beaten appeared to be in their 50s.
Under U.S. pressure, the army is fighting militants in several areas of the northwest, where al-Qaida and Taliban are strong.
Many Pakistanis practice a conservative form of Islam, and opinion polls have shown a deep distrust of the United States. Some are uncomfortable with military action against Muslim extremists and would prefer the government broker deals with them.
But recent polls have suggested a turnaround in public opinion, with strong support for the military offensive in Swat and an increasingly negative view of the Taliban.