Mayor of Hackney Philip Glanville said the council was working with the National Cyber Security Centre (NCSC) to investigate and respond to the incident.
The source of the attack has not yet been identified, but Mr Glanville said the response was focusing on keeping frontline services running, protecting data and "restoring affected services as soon as possible".
A message on the council's website says that it was having a "technical problem" and that people may have difficulty accessing its One Account service – which allows residents to manage their communications with the council – and its online payments system.
"Hackney Council has been the target of a serious cyber attack, which is affecting many of our services and IT systems," Mr Glanville said in a statement.
"Council officers have been working closely with the National Cyber Security Centre, external experts and the Ministry of Housing, Communities and Local Government to investigate and understand the impact of the incident.
Mr Glanville added: "This investigation is at an early stage, and limited information is currently available. We will continue to provide updates as our investigation progresses.
"Our focus is on continuing to deliver essential frontline services, especially to our most vulnerable residents, and protecting data, while restoring affected services as soon as possible.
He Concluded: "In the meantime, some council services may be unavailable or slower than normal, and our call centre is extremely busy. We ask that residents and businesses only contact us if absolutely necessary, and to bear with us while we seek to resolve these issues."
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