The opposition Social Democrats have won control of the Czech senate in mid-term elections, allowing them to slow government plans to cut spending.
Voters were choosing 27 of the 81-seat chamber and the left-wing Social Democrats now have 41 seats.
The gains could allow them to block the ruling right-wing Civic Democrats' plan to send more troops to Afghanistan.
The Social Democrats' leader said their goal was to make the government's reforms "socially more tolerable".
The Civic Democrats formed a coalition government after May's parliamentary elections and now proposes to cut government spending to reduce the country's budget deficit.
The plans, including a 10% cut in the wages of public sector workers, provoked a large protest in Prague last month.
"We are ready to discuss future reforms with the cabinet," said the Social Democrats' acting chairman Bohuslav Sobotka.
"The goal is to make them fair, balanced, socially more tolerable," he said.
Prime Minister Petr Necas said the election result would slow legislation but he was willing to work with the Social Democrats.
Bills passed by the lower house of parliament are sent to the senate for approval before being signed into law by the president. If the senate rejects a bill, it returns to the Civic Democrat-dominated lower house before going straight to the president, bypassing the senate.