On early Friday, Romero reported about 369 arrests.
"As of 8:50 p.m. [00:50 GMT on Saturday], arrests are underway, there are reports about officers of the National Guard and special services rushing into homes. The number of arrested people will soon reach 500," Romero wrote on his Twitter page.
He added that there were at least 14 underage people among those arrested.
The situation in Venezuela, which has been struggling to overcome a political crisis amid the opposition's refusal to recognize last year’s re-election of President Nicolas Maduro, significantly escalated on Tuesday when the opposition-controlled National Assembly declared the president a usurper.
The very next day, opposition-held National Assembly's head Juan Guaido proclaimed himself Venezuela's interim president.
Maduro called the opposition leader a US puppet and accused Washington of attempting to organize a coup in Venezuela, declaring the decision to cut off diplomatic relations with the country.
Some countries, including Brazil, Canada and the United States, immediately recognized Guaido's interim presidency. Other states, such as Russia and Mexico, voiced their support for Maduro as the country's only legitimate president.