Around one thousand people rallied in the capital Bucharest where they also slammed the "incoherence" of government economic policy.
The crowd, comprising chiefly young adults, converged on the headquarters of the ruling Social Democratic party and denounced the party as "thieves".
"We cannot have confidence in leaders -- they want to bring justice under political control," said one protester, 42-year-old computer scientist Iulian Sandu.
Caludiu Dumitrescu, a 35-year-old restaurateur, blasted the government's economic policies which he said he had weakened the leu currency and forced up the price of fuel.
Other marches took place in the western cities of Cluj and Timisoara and the central city of Brasov.
Last week, a similar rally drew some 12,000 people to Bucharest.
Similar protests were also held in dozens of other cities across the country opposing the parliamentary bill that would reduce the powers of the anti-corruption prosecutor's office (DNA) and prevent it from investigating magistrates.
The bill would also mean the president could no longer name senior prosecutors.
Last January, the government tried to soften anti-corruption laws, only to backtrack under pressure after a series of huge protests, the largest since the end of communism in 1989.
The European Commission has warned Romania that its bid to overhaul its justice system could undermine efforts to stamp out rampant corruption.
Romania joined the EU in 2007 and in recent years has been enjoying high growth rates, but both Brussels and the International Monetary Fund have pressed the government to do more to tackle graft.
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