The Metropolitan Police arrested a man suspected of overstaying his visa, but released him after their van was surrounded by a crowd who blocked it from leaving.
Although the gathering on June 11 at Evan Cook Close appeared peaceful, some activists were said to have been pushed by the police.
The Independent Office of Police Conduct (IOPC) urged witnesses to contact the organisation.
“To assist with our investigation into the level of force the police officers used, we would like to hear from anyone who saw what happened," IOPC regional director Sal Naseem said.
“We are aware that footage of the incident, which appeared to show force being used, was posted on social media in the aftermath of the incident and would urge anyone who either has, or knows of, any footage that may have been recorded to contact us as soon as possible.”
One attendee, Eleanor Janega, 39, told the PA news agency at the time of the protest: “We were alerted that there was an immigration raid in process so locals came down to block it.
“We had been sitting here blocking the van and having ice lollies. They brought more police in and tried to break through the crowd by pushing us. We all sat down."
Reginald Popoola, a Labour councillor in Southwark, said protesters had “encircled the van peacefully", but some were shoved during the demonstration.
Anyone with information is urged to contact the IOPC by calling 0300 3035616 or emailing EvanCookeClosepoliceconduct.gov.uk.
The probe was launched after the Met Police made a referral to the IOPC on June 18.