A war memorial is being guarded by police after protestors daubed it with "Free Palestine" graffiti.
Greater Manchester Police (GMP) said the cenotaph was vandalised in Rochdale just after 16:00 GMT.
GMP condemned the vandalism as "criminal disrespect" and Rochdale Council said it was "totally unacceptable".
Pro-Palestinian protests have been held across the UK and the world since the Israel-Gaza war began.
Prime Minister Rishi Sunak has also warned that war memorials could be "desecrated" on Armistice Day.
Police community support officers are now guarding the cenotaph on the Esplanade, Rochdale, after the vandalism.
GMP said there was also another incident of vandalism at the cenotaph on Monday when poppies were taken down.
Ch Supt Nicky Porter said she was "deeply saddened" by the incidents and added that "the memorial is a particularly poignant reminder of those who have given their lives to our country, particularly at this time of year".
"We will not tolerate the criminal disrespect of the town's Memorial Gardens and I hope the action we have already taken in response to these two incidents reassures our communities of that," she added.