(Recasts with judges named)
CARACAS, July 21 (Reuters) - Venezuela's opposition-led congress on Friday appointed alternative judges to the country's Supreme Court, whose current pro-government members have been a bedrock of support for leftist President Nicolas Maduro.
While widely seen as symbolic, the move raises the specter of the development of a parallel state. The top court has warned that the naming of the alternate judges is illegal, and they could be jailed.
Undeterred, opposition lawmakers swore in the 13 new judges and 20 substitute judges in a public plaza to combat what they say is oil-rich Venezuela's slide into dictatorship under Maduro.
"We're not backing down, Venezuela will have a Supreme Court of Justice and institutions at the service of the people and not at the service of whatever government is in power," said opposition legislator Carlos Berrizbeitia during the ceremony, where the appointed justices were applauded and cheered on with shouts of "Bravo!"
Critics hold that the current Supreme Court justices were named illegally by the ruling Socialist Party and rushed in before the opposition took over the legislature in January 2016.
"They're pirate magistrates named on the fly," said opposition legislator Juan Requesens in a video streamed live on the Periscope service, which the opposition often uses given limited coverage of their activities on local television channels.
In a statement broadcast on state television later on Friday, the Supreme Court blasted the alternative judges who were named by the legislature.
"They're undertaking crimes against the independence and security of the nation, in particular, in terms of crimes of treason and against the powers of the nation and states," said Juan Jose Mendoza, the president of the top court's constitutional chamber.
Even so, the government will not allow the congressionally appointed judges to unseat those already sitting on the Supreme Court.
Rather, the move is part of the opposition coalition campaign to ramp up pressure on unpopular Maduro after nearly four months of violent street protests, an unofficial plebiscite against him last weekend and a national strike on Thursday.
Apart from appointing new judges, the opposition held an unofficial referendum on Sunday in which the opposition said some 7.5 million people voted against the government.
Two young men and one teenage boy died in unrest related to the Thursday's strike, according to authorities. Around 100 people have died in unrest that kicked off in early April, thousands have been arrested, and hundreds injured.
Over 360 people were arrested across the country on Thursday, according to the rights group Penal Forum.
Venezuela's second-largest city, Maracaibo, suffered looting and fires during the stoppage, according to local reports that have not been confirmed by authorities.
The opposition is vying to stop Maduro's plan to on July 30 create a controversial super-legislature with powers to rewrite the constitution and supersede other institutions.
Maduro also faces widespread pressure from abroad to abort the assembly, including from U.S. President Donald Trump who said on Monday he would take "strong and swift economic actions" if the Venezuelan leader went ahead with his plans. (Additional reporting by Girish Gupta and Corina Pons in Caracas and Lenin Danieri in Maracaibo; Editing by W Simon)