Neighbours fear they have been targeted after red paint was chucked over their door and inside their letterbox.
Residents of the four-flat property in Lancaster Road, Seven Dials, woke to see paint splattered on the communal door and sinking into the floorboards inside.
And after causing a terrible smell they believe the paint was mixed with excrement.
No-one else on the street suffered the same and the residents have no idea why they have been apparently singled-out.
Royal Sussex County Hospital doctor Gill Jones, who lives in the building, said: “My husband noted a horrible smell in our shared entrance hall at 5.30am and when he opened the front door found gloss red paint on the floor, door and walls of the entrance.
“Our interior carpet has also been taken up since and the floorboards removed to eradicate the smell of the liquid poured through our letterbox. The smell reminds me of when a dog rolls in fox excrement. We do not know what this is about and that makes it worse.
“I have gone out and bought extra smoke alarms – what else might they pour through the letter box - what else is this about?”
The residents have reported the matter to police, who are asking anyone with any information to get in touch.
A Sussex Police spokesman said they are not aware of any similar reports.
Neighbour Jane Currie, who is disabled and housebound, said: “At first we thought it was just red paint but after a little while and especially in this heat it became apparent that there was excrement there as well.
“It was horrendous, really horrendous. I spoke to someone at the council who said this is a 999 call – you are old and vulnerable and it is excrement which is an environmental hazard.”
A Sussex Police spokesman said: “Police were informed that paint had been thrown at a front door of a block of flats on Wednesday.
“The incident is thought to have happened between 7pm on Tuesday, July 15 and 5.30am on Wednesday.
“Police are appealing to anyone who saw what happened or have any information about it to contact police on 101 or email firstname.lastname@example.org quoting serial 482 of 16/7.”