About 100 "indignant" protesters rallied near Spain's parliament Tuesday to protest plans to enshrine a long-term budget cap in the constitution.

A dozen police kept demonstrators a short distance from the parliament's lower house where lawmakers agreed to a fast-track debate on the reform, which could be approved by the Senate as soon as next week.

A slogan -- "No to the constitution change. No to the diktat of the markets" -- was scrawled over one demonstator's umbrella, which shielded him from the August sun.

Others outside the Congress brandished banners reading: "We are the ones paying for this crisis."

"We cannot change the constitution like this. We need a referendum," said a female protester aged about 50 who gave only her first name, Marisa.

The country's biggest unions have joined with citizens' groups to call for protests Wednesday and Thursday across the country and a "major demonstration" in Madrid on September 6.