A banner which appeared to threaten Tories and which was accompanied by hanging effigies is being investigated by police.
The banner, hung from Salford Bridge, said “130,000 killed under Tory rule, time to level the playing field” and was adorned with what appeared to be effigies.
The sign appeared over the River Irwell as the Conservative Party Conference began in the city.
It has since been removed.
No one has yet claimed responsibility for it.
The banner was condemned an “completely unacceptable” by Greater Manchester Labour Mayor Andy Burnham.
Tory councillor Kerry Boyd spotted the sign in Manchester as she went for a pre-conference run.
She said on Twitter: “Does this classify as death/terrorist threat? Utterly vile.”
Ms Boyd is a councillor on Thanet District Council in Kent.