Venezuela’s Supreme Court has ordered “civil and military authorities” to carry out “coercive actions” in response to the appointments, but it is unclear what that will entail.
Meanwhile, the opposition Democratic Unity coalition (MUD) has called for protest marches on July 22 from seven points in the capital Caracas to the Supreme Court headquarters.
The opposition says the current justices are illegitimate, having been rushed into their positions shortly before the governing party lost its majority in 2015.
Since the opposition took over the National Assembly last year the court has consistently blocked all bills passed by Congress.
The opposition announced last week that it would appoint new judges and that it would also take the first steps to set up a national unity government. Analysts say such proposals raise the possibility of a parallel state structure.
On July 20, millions of Venezuelans joined a general strike called by the opposition.
At least three people were killed in clashes between police and protesters and there were more than 300 arrests.
Protesters barricaded roads in Caracas and other cities with rubbish and furniture.
The opposition said that 85% of the country joined the strike but President Maduro said its effect was minimal and that its leaders would be arrested.
Meanwhile, Colombia, France, Spain, the US and the EU have urged the Venezuelan government to cancel the vote for a new constituent assembly on July 30.
However, President Nicolas Maduro has rejected the calls.
The first step in Venezuela’s opposition campaign to recall President Nicolas Maduro has been approved, the national election council has announced.
The council said the opposition had succeeded in gathering 1% of voter signatures in all 24 of Venezuela’s states.
The move is the first part of the opposition’s push for an early end to Nicolas Maduro’s term in office.
In a further twist, the Supreme Court suspended opposition activity in Venezuela’s parliament.
The court said activity would be frozen until three opposition members being investigated for vote-buying were removed.
The country is going through a political and economic crisis, which has led to shortages of basic goods and looting.
The inflation rate is one of the highest in the world and there are long queues outside shops.
The election council said Nicolas Maduro’s opponents had cleared the threshold of obtaining 200,000 valid signatures on a petition demanding that the president face a recall referendum.
The council did not set a date for the next stage of the lengthy recall process – when the opposition will need to collect four million signatures in just three days.
The opposition accuses Nicolas Maduro’s administration of mismanaging the economy.
He was elected in April 2013 and his term runs until 2019.
Correspondents say election council head Tibisay Lucena provided the president with a major fillip by stating that claims of widespread fraud in the opposition petition should be investigated.
Tibisay Lucena said the authorities had detected more than 1,000 apparently falsified signatures.
“The electoral authority will ask the state prosecutor’s office to investigate,” she said.
However, Tibisay Lucena made clear that 98% out of about 408,000 signatures gathered by the opposition Democratic Unity (MUD) coalition – twice the minimum required in the initial phase – had been validated.
The government made clear that it was determined not to allow a referendum this year.
It has initiated nearly 9,000 lawsuits around the country in an effort to try to halt the referendum push.
Correspondents say timing is vital because if President Nicolas Maduro loses a referendum this year – as polls suggest he will – a new presidential vote will be triggered, giving the opposition a chance to end 17 years of socialist rule.
If the president loses a referendum in 2017, he would be replaced by his vice-president, effectively ensuring the socialist party remains in power until the next presidential election scheduled for 2018.
Opposition leaders want Tibisay Lucena immediately to announce a date for the collection of 20% of signatures in order to trigger a referendum as soon as possible.