US announces sanctions against Venezuelan President Maduro The sanctions were imposed on Maduro after the U.S. called the vote a "sham."

 -- The U.S. Treasury Department has sanctioned Venezuelan President Nicolas Maduro just one day after his government held elections for a constituent assembly that would rewrite the country’s constitution.

Today's decision by the Treasury freezes all of Maduro’s assets that “are subject to U.S. jurisdiction” and prohibits all “U.S. persons” from doing business with him. He is the latest official in his government to be sanctioned by the Trump administration.

Treasury Secretary Steve Mnuchin blasted Maduro as a “dictator” and said that his “illegitimate” election proved that the leader “disregards the will of the Venezuelan people.”

“By sanctioning Maduro, the United States makes clear our opposition to the policies of his regime and our support for the people of Venezuela who seek to return their country to a full and prosperous democracy,” said Mnuchin.

Mnuchin also warned that “anyone who participated in this illegitimate ANC” could be exposed to future U.S. sanctions for their role in “undermining” Venezuelan democracy.

The sanctions came as the Maduro government held elections for the National Constituent Assembly, or ANC, on Sunday. The election allowed voters to choose representatives who would rewrite the constitution and, critics say, consolidate power in Maduro’s hands.

Maduro said on Monday that he was proud of the today’s sanctions. He also called Trump an emperor, and added that the sanctions are a sign of "[Trump’s] helplessness, his desperation and his hatred."

Maduro added that Trump’s biggest mistake in life will be messing with Venezuela.

"I don't obey imperial orders... I'm against the Ku Klux Klan that governs the White House, and I'm proud to feel that way,” the Venezuelan leader said at the headquarters of the national election commission.

The vote Sunday followed four months of political upheaval, with more than a 120 people killed since April. 15 more were killed Sunday, including the opposition’s youth secretary in Sucre, according to the Associated Press.

The U.S. government has developed a contentious relationship with Maduro over the past months.

Nikki Haley, the U.S. ambassador to the United Nations, called the constituent assembly election “a sham” and “another step towards dictatorship” on Sunday and warned the U.S. wouldn’t “accept an [illegitimate] govt” on Twitter.

Maduro’s sham election is another step toward dictatorship. We won't accept an illegit govt. The Venezuelan ppl & democracy will prevail. — Nikki Haley (@nikkihaley) July 30, 2017

Maduro made it clear on state-run TV Sunday that he wanted to strip opposition legislators of their constitutional immunity from prosecution and jail one of them, even taking to calling one of them “little Hitler.” Overnight, a candidate for the assembly, Jose Felix Pinada, was killed by a group of people who burst into his residence and opened fire. It was unclear who was behind his death.

Venezuela’s political opposition holds a majority in the country’s legislature, and they have fought back against Maduro’s rule. The constituent assembly is seen by the opposition as his way to circumvent the democratically elected body.

The Trump administration has tried to bolster the opposition as it seeks to challenge Maduro's government and prevent it from spiraling further into violence.

Vice President Mike Pence spoke with opposition leader Leopoldo Lopez about the crisis on Friday and demanded the “full and unconditional release of all political prisoners in Venezuela.” After more than three years in prison, Lopez was released earlier this month and is now under house arrest.

On July 26, the Treasury Department imposed sanctions on 13 current and former Venezuelan officials in a bid to deter the president from holding the ANC vote.

Key Maduro allies were sanctioned, such as President of the National Electoral Council Tibisay Lucena Ramirez, Venezuela’s Ombudsman Tarek William Halabi, Commander General of the National Guard Sergio Marcano, and head of the Presidential Commission for the National Constituent Assembly Elias Jose Milano.

That round of sanctions came after the opposition held a symbolic referendum on July 16 in which more than 7 million people voted against holding a constituent assembly vote, the AP reported.

The Venezuelan government reported that more than 8 million Venezuelans voted in the ANC elections Sunday, but independent monitors disputed that figure.

The Venezuelan opposition estimates that up to 15,000 civilians have been wounded in recent protests, with over 3,000 people arrested and 431 political prisoners in detention without fair and transparent judicial process.

Nicolas Maduro was elected President of Venezuela on April 14, 2013, following the death of former President Hugo Chavez. Maduro held previous roles in the Venezuelan government, including as Executive Vice President and Minister of Foreign Affairs.

ABC News’ Benjamin Gittleson contributed to this report.