"This is a death cult. The Texas GOP. Only they want you to do the dying," the former Democratic presidential contender said.
Several states are reversing or halting re-opening efforts, as coronavirus cases surge.
The mayor of Phoenix said it feels like the federal government is "declaring victory while we're still in crisis mode."
Florida's coronavirus cases rose by a record 11,458 on Saturday, the state’s health department said.
The U.S. recorded 51,200 new confirmed cases Wednesday, according to a tally kept by Johns Hopkins University
The Republican governor, who had said he did not believe in mandatory mask rules, urged Texans to wear face coverings or risk fines of up to $250.
Several attendees who were at the June 20 event have tested positive for the coronavirus, and the total number of cases in Lakeway is expected to increase.
As COVID-19 cases soar to new highs, governors of several states, including Texas, Florida and Arizona, are backtracking on their reopening plans.
Arizona joins Texas in holding off on reopening efforts amid the surge of COVID-19 cases.
The state was previously on an aggressive path to reopening after coronavirus-related closures.