The country's top PSI 20 equity index fell more than 3% at one point after opening down 0.6%. The broader European equity market was broadly unchanged.

Portuguese government bonds pared some of their price gains for the day and underperformed their peers.

The 10-year bond yield was down 2.3 basis points at 3.40%, while 10-year bond yields in Italy and Spain were 9 and 6 bps lower respectively. Yields move inversely to prices.

The closely-watched gap between 10-year government bond yields in Portugal and euro zone benchmark bond issuer Germany widened to 69 bps, from 65 bps at Monday's close.

"Portugal's been quite market friendly from a political perspective... This might not change things, but there is a bit of a sell-off in Portuguese bonds given that the PM's just resigned very suddenly over a corruption scandal investigation," said Lyn Graham-Taylor, senior rates strategist at Rabobank.

Shares in Galp Energia led stock fallers in Lisbon, down more than 5%.

(Reporting by Danilo Masoni, Joice Alves and Harry Robertson; Editing by Dhara Ranasinghe)