Police were called to reports that a child was in the River Irwell in Radcliffe shortly before 4.25pm on Wednesday.
The child, whose age has not been confirmed, was rescued after emergency services attended the scene and was taken to hospital where he is in a critical condition.
A police cordon is in place in Blackburn Street surrounding a town centre bridge.
Police are currently responding to an incident in #Radcliffe. Shortly before 4.25pm today (Wednesday 11 September 2019), police were called to reports that a child was in the River Irwell. pic.twitter.com/3QKKDJXWhX— Greater Manchester Police (@gmpolice)September 11, 2019
The boy was reportedly thrown into the water from a bridge. Eyewitnesses said that two fire engines, ambulances, multiple police vehicles and the GMP force helicopter were at the area, according to the Manchester Evening News.
Greater Manchester Police tweeted: "Police are currently responding to an incident in #Radcliffe."
"Shortly before 4.25pm (Wednesday 11 September 2019), police were called to reports that a child was in the River Irwell."
A spokeswoman for Greater Manchester Fire and Rescue Service said: "We were called at 4.28pm to a rescue of a person in water in Pilkington Way, Radcliffe.
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"Fire engines from Whitefield, Farnworth and Eccles attended the incident along with a water incident unit from Eccles.
A spokeswoman for Greater Manchester Police said: "A man has been arrested on suspicion of attempted murder. He remains in custody for questioning.
"An investigation is under way and enquiries are ongoing."
Anyone with information is asked to contact police on 101 quoting incident number 1930 of 11/09/2019, or Crimestoppers, anonymously, on 0800 555 111.