There’s no national law on the subject, but Portuguese towns are free to pass their own rules banning the feeding of stray animals; the town of Oeiras has a schedule of fines that goes all the way up to €8,000.
In most places, the regulations come under health and sanitation rules, and are intended to avoid both disease and unsightly or smelly roadside litter; in some cases there’s also an intent to limit the growth of stray populations.
Among the localities where feeding feral animals is an offense of Lisbon, Porto, Cascais, and Sintra, as well as the extreme rule in Oeiras, where the fines start at €50 before reaching the top number for the most serious or repeated offenses.
