What does it mean in other words?
at the hands of

the context

The Awamiya residents are said to have been convicted of sedition, attacks on security personnel and interference in neighboring Bahrain. According to the BBC, since 2011, Awamiya has been the centre of protests by Saudi Arabia’s Shia minority, which has long complained of marginalization at the hands of the Sunni-controlled monarchy.

Question

What does it mean in other words?
at the hands of

the context

The Awamiya residents are said to have been convicted of sedition, attacks on security personnel and interference in neighboring Bahrain. According to the BBC, since 2011, Awamiya has been the centre of protests by Saudi Arabia’s Shia minority, which has long complained of marginalization at the hands of the Sunni-controlled monarchy.

Answer

'as done by' or 'committed by' or 'from'
simplified as:
"Which has long complained of marginalization from the Sunni-controlled monarchy"