The protesters, some waving Lebanese army flags and wearing camouflage trousers, breached coils of barbed wire as they sought to approach the parliament building where MPs were voting on the budget.
Lebanon, one of the world's most heavily indebted states, is seeking to slash the deficit with the aim of steering the public finances towards a sustainable path.
The draft budget submitted by the government to the parliament included a cut in veterans' benefits and a freeze on early retirement from the military.
Parliament has however been reviewing the budget and the final articles concerning army retirees are not yet clear.