Millions of Portuguese will cast their ballots on Sunday in a third general election in as many years, though many are bracing for more uncertainty as the vote is unlikely to deliver a stable government.

Portugal's general election campaign ends on Friday for a vote that Prime Minister Luis Montenegro is expected to win, but with no guarantee he can form a more stable government.

Portuguese political parties were wrapping up their campaigns on Lisbon's streets on Friday ahead of a general election on Sunday - the country's third early vote in as many years - that polls indicate is unlikely to yield a stable government.