Trump vows full support at mass anti-abortion rally

US President Donald Trump became the first sitting president to attend an annual anti-abortion rally in Washington, hailing himself as a staunch advocate of the movement, in a speech peppered with religious references.

"Unborn children have never had a stronger defender in the White House," Trump told the thousands in the crowd on the National Mall. "Every child is a precious and sacred gift from God."

The March for Life has been a fixture since the 1970s, but no president ever attended the event, which is focused on one of the most divisive topics in US political life.

The right to have an abortion is protected under a 1973 Supreme Court ruling, known as Roe vs Wade, but conservatives have been trying to claw back absolute protections, including by placing restrictions on services at the state level and cutting federal funding for clinics.

Trump's appearance comes as he gears up for a re-election campaign and looks to lean on Christian-conservatives as part of his base.

Trump attended the rally, just south of the White House and down the road from the Capitol, as the Senate was preparing to restart opening arguments in the impeachment trial against him.

"They are coming after me because I am fighting for you, and we are fighting for those who have no voice, and we will win, because we know how to win," he told the cheering crowd.

Trump's election in 2016 sparked concern among women's rights activists that he would significantly go after abortion, including through judicial appointments.