Mr Maduro gave US diplomats 72 hours to leave the country.
On Wednesday Mr Guaidó declared himself president during mass protests. The US said "former President Maduro" no longer had authority and urged the army to back Mr Guaidó instead.
So far the army has remained loyal to Mr Maduro.
Venezuela has been in economic freefall under President Maduro. Hyperinflation, power cuts and shortages of basic items have driven millions of people from the country.