US staff left the embassy earlier this year as tensions between Washington and Caracas escalated after Venezuelan opposition leader Juan Guaido proclaimed himself an interim president and was backed by the United States despite criticism from Maduro and Venezuelan authorities.
"I have ordered to boost the legal police protection of the building of the former US embassy which belongs to the US government. We will protect it because Venezuela follows conventions and international law. Criminals are sitting in the White House," Maduro said on Thursday.
On Monday, US Secret Service agents broke into the embassy and ordered the activists to leave the embassy or face imprisonment and prosecution. Earlier on Thursday, US Secret Service and State Department police forced their way into the embassy and detained the four activists who had ignored the threats and remained there, both federal agencies confirmed.