Video of man tased from behind as he sat on curb sparks 'great concern,' mayor says

The mayor of Lancaster, Pennsylvania, said an investigation is underway after a viral video on social media showed a Lancaster police officer tasing a black man in broad daylight.

“Like you, when I saw the video I was upset by it and it is of great concern to me,” Mayor Danene Sorace said Friday in a video posted to her Facebook page. “We take the use of force very seriously. There is an investigation that has already commenced.”

On Thursday, Lancaster police responded to a 911 call that said a man had gone after four people with a bat, according to authorities.

According to the police report, Officer Shannon Mazzante responded to the call and encountered a group of three individuals "telling a male to get away from them." Police said Mazzante told the man to sit down "several times" but he did not listen.

The man, who was identified as Sean Williams, would not leave a woman in the group alone and "kept repeating that he wanted a specific item, his Social Security card, from her," police said.

Shortly after, Officer Philip Bernot arrived on the scene and warned Williams that a Taser would be used if he did not listen, police said. In video taken by an eyewitness, Mazzante and Bernot can be heard telling Williams to sit down on the curb.

"Legs straight out or you're getting tased. Straight out! Straight out!" Bernot said.

"Put your legs straight out and cross them now!" Mazzante added.

Bernot then deployed the Taser and took Williams into custody. Officers said Williams had "failed to follow instructions," according to the report.

"Williams was instructed to stick his legs straight out in front of him and to cross his ankles. This is done as a measure of control to insure that if someone is going to flee or offer physical resistance, they will have to move their legs under them to do so. Non-compliance is often a precursor to someone that is preparing to flee or fight with officers," the report said.

Juan Almestica, the bystander who took the video, told ABC News Friday that he went outside after hearing a commotion.

"One of the officers is telling him to put his legs straight and another one is telling him to cross his legs. ... Then they tased him. ... I don't have any police training but I could have handled that situation better myself," Almestica said.

Bystanders told police Williams "had been exhibiting increasing erratic behavior over the previous few days."

They did not mention that he was carrying a bat.

Police said Williams, who was charged with a criminal warrant for possession controlled substance and public drunkenness, was checked and cleared by EMS "as is protocol with every use of an ECD (electronic control device)." He was later released on unsecured $5,000 bail.

In her Facebook post, Sorace said her team was already in contact with civil rights groups to address their concerns.

Blanding Watson, president of the Lancaster NAACP, told the Lancaster Online that the incident "requires a full investigation."

ABC News' Fergal Gallagher contributed to this story.