Thousands turned out to voice their opposition to the plans by Dublin City Council, which would see a series of earthen embankments and walls being built between Alfie Byrne Road and the Bull Wall.
Gardai told the Clontarf Residents' Association as many as 5,000 turned out.
The plans were described as "ludicrous" and "dangerous" by speakers as they petitioned the council to scrap the designs. But locals fear the local authority remains determined to implement the scheme, which has the approval of An Bord Pleanala.
Public representatives and families in the upmarket suburb claim they were never properly consulted.
The council is now preparing a report on how to protect properties in Clontarf from coastal flooding.
The protest was supported by city councillors and TDs, including Government minister Richard Bruton, Labour deputy Aodhan O Riordain, Independent TD Finian McGrath and former Lord Mayor Gerry Breen (FG).
Speaking near Clontarf's Wooden Bridge, Mr Bruton expressed disappointment at the way the plans were dealt with, while Mr McGrath drew attention to the security implications of cutting off long sections of the walkway from public view.
Clontarf is home to a string of well-known faces, including RTE's Joe Duffy, Fine Gael minister Richard Bruton and former Fianna Fail senator Ivor Callely.
The late Gerry Ryan grew up and lived locally until he split with his wife Morah, while Irish rugby captain Brian O'Driscoll and Marty Whelan are also natives.
The project makes provision for mounds and walls averaging just less than 2m in height stretching 3km along the suburb's promenade.
It is to raise the level of the ground by up to 2.75m in some places.
Residents say they only became aware of the designs in recent days, as the council prepared to sign contracts to construct the scheme.
"If implemented, the sea view in Clontarf will be eliminated," the Clontarf Residents' Association (CRA) and Clontarf Business Association (CBA) said, adding that "pedestrians or joggers using the pathway close to the sea will not be able to see the road".
"Equally importantly they will not be visible from the road which produces its own potential security risk.
"Protecting our homes and businesses from flooding is vital. The CRA and the CBA were aware that these defences were being planned in the form of a mound or wall. However, both the CRA and the CBA were under the impression that the height would be less than one metre.
Initially, city chiefs defended themselves, saying residents were written to in advance of the planning phase and informed of the plans.
However, council chiefs now accept they had not communicated properly with locals.
The promenade is not level so the defences vary from 0.85m (2ft10in) to 2.75m (9ft), reaching a uniform height.
The EIS gives the average height as 1.7m (5ft 6in). "These are generally gently sloped grassed mounds and not walls," assistant city manager Seamus Lyons said.
The scheme has been designed to prevent flooding to seaside homes and businesses as well as to carry a new arterial water main.
City engineers have allocated in the region of €9.7m for the overall water main project, with around €4m being spent on the defence system.