The video below, released by state broadcaster Rai on Monday, shows migrants lining up and stripping before being hosed down.
They are disinfected in an effort to combat scabies, which none of the migrants had on arrival on the island, Rai said.
Men and women are subjected to the same treatment, according to the unnamed migrant who took the footage. People are treated “like animals”, he said after spending more than two months at the reception centre for new arrivals.
“You think - this is Italy,” he said.
Responding to the footage, Lampedusa Mayor Giusi Nicolini said it was like a “concentration camp”.
“Lampedusa and Italy must be ashamed of this model of welcome. It needs to change,” Nicolini told Rai.
The EU's Home Affairs Commissioner Cecilia Malmström said on Wednesday that investigations into the "appalling conditions in many Italian detention centres, including Lampedusa" are underway.
Laura Boldrini, president of the Chamber of Deputies, said it was “shameful for a civil country”.
“We cannot remain indifferent. This degrading treatment throws our country into strong disrepute,” said Boldrini in an online statement.
An investigation into the incident has now been opened, Italian media reported on Wednesday.
The video comes two months after Boldrini visited Lampedusa with Prime Minister Enrico Letta and José Manuel Barroso, president of the European Commission, following the death of more than 400 migrants in two shipwrecks off the island.
On Tuesday, the Italian coast guard rescued 110 people from a dingy off the coast of Lampedusa, the latest of around 33,000 migrants that have arrived in Italy so far this year.
Watch the video from the Lampedusa reception centre: