Hillary Clinton compared President Donald Trump to the president of Turkey - a controversial figure who returned to office with broad new powers that lack checks and balances.

In a speech to Oxford University on Monday, Clinton again bemoaned the American electoral college system that saw her win the popular vote in 2016 but lose the presidency.

'Populists can stay in power by mobilizing a fervent base. Now, there are many other lessons like this, she said, adding that she had 'my personal experience with winning three million more votes but still losing.'

Hillary Clinton delivers the Romanes Lecture at the Sheldonian Theatre, Oxford

She compared President Donald Trump to the president of Turkey - a controversial figure who returned to office with broad new powers that lack checks and balances

She fretted people 'underestimate the threat' leaders like Trump and Recep Tayyip Erdogan 'pose to the survival of democratic institutions'

'And we will leave discussions of American Electoral College for another day,' Hillary noted during her Oxford University's Romanes Lecture.

Then she quoted liberal academic Yascha Mounk, who studies democracy.

'But Mounk concludes by saying, 'Turkey also shows that political and intellectual elites, both inside the country and around the world, persistently underestimate the threat which these kinds of leaders pose to the survival of democratic institutions,'' Clinton said.

'We are in the midst of a global struggle,' Clinton said, 'between liberal democracy and a rising tide of illiberalism.'

Her reference to Turkey to its President Recep Tayyip Erdogan, who has been criticized as having authoritarian behavior and rejecting democratic principles.

Turkish voters reelected Erdogan on Sunday with extensive new executive powers in his hand and with the post of prime minister abolished.

Defeated opposition candidate Muharrem Ince said Turkey was now entering a dangerous period of 'one-man rule,' according to the BBC.

Erdogan won by 20 points in a fiercely fought election.

Hillary Clinton arriving to deliver the Romanes Lecture at the Sheldonian Theatre, Oxford

Hillary Clinton delivers the Romanes Lecture at the Sheldonian Theatre, Oxford

While he has helped Turkey's economy get stronger, he has also cracked down on opponents and put some 160,000 people in jail.

Some critics argue the new powers - approved last year but put in power this year - place too much power in one person's hands, and that Turkey's new system lacks the checks and balances of other executive presidencies like the United States.

Erdogan argues his increased authority will empower him to address Turkey's economic woes and defeat Kurdish rebels in the country's south-east.

Erdogan was prime minister for 11 years before becoming president in 2014. Under the new constitution, he could stand for a third term when his second finishes in 2023, meaning he could potentially hold power until 2028.