Venezuela's electoral council has suspended the opposition's campaign to hold a recall referendum against President Nicolas Maduro.
The council said that courts in several states had reported fraud in the campaign's preliminary petition.
The opposition, which planned to secure the required signatures for the vote next week, condemned the move, saying it would escalate the crisis.
Eight opposition heads have reportedly been barred from leaving the country.
"In adherence to the constitution, the National Electoral Council abides by the decisions ordered by the tribunals and has sent instructions to postpone the process of signature gathering until new judicial instructions are known," the council said in a statement.
Regional courts in at least four states had ruled that there was fraud during the first round in June, when the opposition collected signatures from 1% of the electorate.
To trigger a recall referendum, the opposition would have needed to get petition signatures from 20% of the country's voters - or four million people - in each of the nation's 24 states at the next stage of the process.
Opposition leader Henrique Capriles warned that the authorities were now "pushing toward a very dangerous scenario in which the crisis worsens".
He later said on Twitter that he and seven other opposition heads had received court orders barring them from leaving the country.
Jesus Torrealba, the head of the Democratic Unity Roundtable (MUD), the main opposition coalition, was also barred, Mr Capriles said.
The timing of council's move is crucial because, according to the constitution, a vote to recall Mr Maduro this year would trigger a presidential election that polls indicate the opposition is likely to win.
But if the president is defeated in a vote next year, the vice-president would replace Mr Maduro and the Socialists would thus remain in power.
The opposition accuses the government of dragging its feet while not totally rejecting the recall referendum process.
Venezuela is suffering a severe economic crisis which the opposition blames on President Maduro.
He says the economic crisis and efforts to get rid of him are a capitalist conspiracy.
Mr Maduro has launched legal challenges against the referendum drive and has vowed there will be no referendum this year.