Portuguese bonds and stocks fell heavily in early trading Wednesday after the resignation of Foreign Minister Paulo Portas, the leader of the junior party in the governing coalition, triggering the worst political crisis since Portugal accepted an international bailout two years ago.

Portuguese bonds were already on a weaker footing before Mr, Portas' resignation; they had dipped Tuesday after the surprise resignation of Finance Minister Vitor Gaspar. Wednesday, the selloff accelerated sharply.

Portuguese 10-year bond yields shot above 7%, widely seen as an unsustainably high level, amid fears Mr. Portas's Democratic and Social Center Party will withdraw its support for the government. Yields pushed higher in other peripheral euro-zone countries as fears of contagion grew. Bond yields rise as prices drop.

"The political problems increase the uncertainty surrounding Portugal's bailout commitments and potentially even the prospect for negotiations of a precautionary program succeeding the current program running out in May next year," RBC said in a note to clients. "We see the risk of further spillover effects into Spanish bonds and Italian bonds hampering the recent recovery."

Ten-year Portuguese yields rose more than a percentage point to 7.525%. Yields on Italian 10-year bonds rose by 0.19 percentage point to 4.589%, while Greek bonds were quoted up 0.15 percentage point at 11.065% according to Tradeweb.

Portugal's benchmark equity index, the PSI-20, slumped by over 6% at the open. This has also weighed on the core European markets, with Germany's DAX index down 1.5%, the CAC-40 in France off 1.4%, and in the U.K. the FTSE 100 index 1.1% lower.

Portugal's banking stocks have been especially hard hit, with Banco Espirito Santo SA (BES.LB) dropping 7.7%, Banco Comercial Portugues SA (BCP.LB) down almost 13%, and Banco BPI SA (BPI.LB) slumping 9%.

Mr. Portas leads the conservative Democratic and Social Center Party, which the government relies on for its majority. It isn't clear whether his party will withdraw its support for the center-right coalition government.

He stood down a day after Finance Minister Vitor Gaspar, the architect of the country's austerity plan under its EU-IMF bailout, also quit, triggering calls for an early election.

In his resignation letter, Portas said he disapproved of the prime minister's appointment of Treasury Secretary Maria Luis Albuquerque to replace Gaspar, highlighting divisions in cabinet about the future of the government's austerity plan.

Like Mr. Gaspar, Ms. Albuquerque has emphasized a need for the government to keep tight control over its budget. Her appointment was considered unlikely to lead to a strong shift in policy.

Prime Minister Passos Coelho said late Tuesday in a televised address that he hadn't accepted the resignation, citing the likely political instability. "I won't resign and won't abandon my country," he said.

Compared with Greece, another country pushing through unpopular cost-cutting measures, Portugal's government was previously united behind austerity measures. In recent months a series of strikes and street protests in Portugal have tested the limits of the public's tolerance for cost-cutting.

(Patricia Kowsmann and Peter Nurse contributed to this article.)

Write to Ed Ballard at ed.ballard@dowjones.com