Officials said the men "returned compliantly to prison when asked to do so".
The Prison Service said it had now strengthened its processes.
Justice Secretary Robert Buckland had previously said rigorous checks would take place before inmates were let out on the scheme.
Up to 4,000 prisoners - those who were due to be freed within two months anyway - were eligible for the early release programme across England and Wales.
Officials said on Tuesday that hundreds would be freed by the end of the week - with 14 pregnant prisoners and mothers with babies among those released.
The six men were released mistakenly from Sudbury and Leyhill prisons and Isis young offenders institution in south-east London.
For full article https://www.bbc.co.uk/news/uk-52334848