Brazilian voters have delivered an unexpected setback to president Luiz Inácio Lula da Silva, who had been widely expected to win an outright majority in elections on Sunday and avoid the need for a second-round run-off on October 29.

With a little more than 98 per cent of the vote counted, electoral authorities ordered a run-off between Mr Lula da Silva and Geraldo Alckmin of the centrist opposition PSDB.