Donald Trump has told pro-lifers marching in Washington, DC that he will veto any legislation that 'weakens the protection of human life'.

The US President made the bold promise in a message broadcast to the March for Life rally against abortion on Friday.

'When we look into the eyes of a newborn child we see the beauty and the human soul and the majesty of God's creation,' he said.

'We know that every life has meaning and that every life is worth protecting.

'As President, I will always defend the first right of our Declaration of Independence, the right to life.' 

Trump has moved to strengthen pro-life protections in the US since coming to office two years ago, including reinstating the Mexico City Policy which prevents US taxpayer dollars from being used to fund organisations or programs that support abortion. 

He has also backed Little Sisters of the Poor in their legal battle to be exempt from offering birth control and sought to remove Title X funding from clinics that provide abortions. 

In addition, the President has vowed to uphold the Hyde Amendment banning the use of federal funds for abortion, something the Democrats said this week they would seek to repeal. 

Addressing thousands of people on the National Mall, Trump promised to veto all pro-abortion legislation. 

'Today I have signed a letter to Congress to make clear that if they send any legislation to my desk that weakens the protection of human life I will issue a veto,' he said.

'We have the support to uphold those vetoes. Every child is a sacred gift from God.'

Vice President Mike Pence also addressed the rally, accompanied by his wife Karen. 

'We know in our heart of hearts that life is winning in America once again,' he said, drawing cheers from the crowd. 

He added, 'To all the pro-life elected leaders gathered here today, we say from the bottom of our hearts "thank you".  Thank you for your stand, thank you for serving for life.'