The U.S. imposed sanctions against Venezuelan President Nicolás Maduro on Monday, saying his government abused human rights and organized an illegitimate vote designed to advance an authoritarian regime, as the leader threatened his domestic opponents with imprisonment.

The U.S. Treasury Department’s move freezes any assets Mr. Maduro has within American jurisdiction, putting him in a small club of leaders it targets including North Korea’s Kim Jong Un, Zimbabwe’s Robert Mugabe and Syria’s Bashar al-Assad.

It was unclear if Mr. Maduro had any U.S.-linked assets but the designation bans his access to the U.S. financial and commercial markets and prohibits any American entity from conducting business with him.

Caracas said 8.1 million people voted on Sunday to choose delegates to form an assembly to write a new national charter. The results drew scorn from many Venezuelans and condemnation from governments in Europe and the Americas who say the assembly will give the government unchecked authority.

The results were a foregone conclusion, since voters had been asked to choose 545 delegates from 6,000 candidates handpicked by the ruling party. Critics of the vote said it also was plagued by a lack of independent observers and safeguards to prevent people from casting multiple ballots.