CARACAS, Venezuela — Reeling from American oil sanctions, President Nicolás Maduro’s government on Tuesday fought to keep its grip on Venezuela, opening an investigation into what it called the “violent acts” of the nation’s opposition leader, freezing his assets and barring him from leaving the country.

The announcement came only hours after the Trump administration said that it had handed control of Venezuela’s bank accounts and property in the United States to the opposition leader, Juan Guaidó, hoping to give him the tools to start running the country.

It was one of Washington’s most overt attempts in decades to carry out regime change in Latin America. In recent days, the Trump administration has recognized Mr. Guaidó as Venezuela’s legitimate president and imposed sanctions to cut off Mr. Maduro’s access to a major source of the nation’s cash: oil sales in the United States.

Now the fierce struggle to shape Venezuela is entering a new, uncertain phase. For many in the capital, the only question about Washington’s aggressive bid to oust Mr. Maduro and help install an opposition-led interim government is, what took so long?