President Nayib Bukele on Sunday secured a thumping victory in El Salvador's elections after voters cast aside concerns about erosion of democracy to reward him for a fierce gang crackdown that transformed security in the Central American country.

Thousands of Bukele's supporters clad in cyan blue and waving flags thronged San Salvador's central square to celebrate his re-election, which the 42-year-old leader termed a "referendum" on his government.

Bukele declared himself the winner before official results were announced, claiming to have attained more than 85% of the vote. Provisional results showed Bukele winning 83% support with 31% of the ballots counted.

His New Ideas party is expected to win almost all of the 60 seats in the legislative body, tightening its grip on the country and bestowing even more sway on Bukele, the most powerful leader in El Salvador's modern history.

"All together the opposition was pulverized," Bukele, standing with his wife on the balcony of the National Palace, told his supporters.

"El Salvador went from being the most unsafe (country) to the safest. Now in these next five years, wait to see what we are going to do," Bukele added.

New Ideas' electoral success means Bukele will wield unprecedented power and be able to overhaul El Salvador's constitution, which his opponents fear will result in scrapping of term limits.

Wildly popular, Bukele has campaigned on the success of his security strategy under which authorities suspended civil liberties to arrest more than 75,000 Salvadorans without charges. The detentions led to a sharp decline in nationwide murder rates and fundamentally altered a country of 6.3 million people that was once among the world's most dangerous.

But some analysts have said the mass incarceration of 1% of the population is not sustainable long-term.

Hours earlier, bullish Bukele held a press conference and said his party needed all the support it could muster to maintain its anti-gang fight and continue reshaping El Salvador.

"So, if we have already overcome our cancer, with metastases that were the gangs, now we only have to recover and be the person we always wanted to be," said Bukele.

Few doubted the outcome of the elections. Polls showed most voters wanted to reward Bukele for decimating the crime groups that made life intolerable in El Salvador and fueled waves of migration to the United States.

Guadalupe Guillen, a 55-year-old shopkeeper, showed up at the victory party dressed in a tunic and Arab scarf, a nod to Bukele's Palestinian family heritage.