President Pena Nieto has branded Donald Trump's policies 'a real threat' to Mexico as he ramps up rhetoric against the Republican a day after the pair met for talks.

While Nieto and Trump struck a conciliatory tone at a joint press conference yesterday, both have since gone on the attack.

Nieto in particular has sought to toughen his stance after the meeting was dubbed a 'national humiliation' and he was branded a 'traitor' by protesters saying he should have forced Trump to apologize for his comments on Mexican immigrants.

Speaking to young people during a late-night town hall on Thursday, Nieto defended the meeting, saying it was a tough decision, but one he felt it necessary to take.

He said the easier path would have been to 'cross my arms' and do nothing in response to Trump's 'affronts, insults and humiliations.'

Nieto added that he believes it necessary to open a 'space for dialogue' to stress the importance of the U.S.-Mexico relationship.

Demonstrators are also enraged by the fact that he did not push back on Trump's policy to build a wall along the southern US border.

During a joint press conference Trump also claimed that the pair had not discussed who would pay for the wall - before later insisting that Mexico would foot the bill 'even if they don't know it yet.'

Nieto has since said that the first thing he told Trump during the meeting was that Mexico would not pay for the wall.

However, many have questioned why the President, who speaks good English, did not correct Trump at the time despite being stood just feet away. 

Former Mexican President Vicente Fox, Nieto's predecessor who previously said 'there is no f***ing way' Mexico will pay for the wall, has branded Nieto a 'traitor'.

He told CNN's New Day: 'I think it's nothing more than a political stunt. Trump is using Mexico, is using President Peña to boost his sinking poll numbers. 

'I think that President Peña is taking an enormous political risk by hosting Trump. If he's perceived as going soft on Trump it will hurt him greatly.

'He will even be considered like a traitor.' 

Meanwhile Trump delivered a blistering immigration speech in Arizona on Wednesday night in which he vowed to make Mexico pay for the wall.

Referring to the Mexican government, he said: 'They're great people, great leaders, but they're going to pay for the wall.'

'On day one, we will begin working on an impenetrable, physical, tall, powerful, beautiful southern wall,' Trump boasted.

He pledged to employ technologies to harden the barrier, including 'above and below ground sensors' along with 'towers, aerial surveillance and manpower.'

This, he said, would 'supplement the wall, find and dislocate tunnels and keep out criminal cartels.'

Trump said that after meeting with Nieto, he trusts 'Mexico will work with us. I really believe it.'

'They will work with us, they really will. They want to solve this problem with us,' he said of the drugs, guns, money and illegal immigrants who cross the border.