Pope Francis has tackled divisive issues such as immigration and the death penalty, as he became the first pontiff to address a joint meeting of Congress.

US Supreme Court justices, the President's Cabinet and diplomats have joined hundreds of lawmakers in the packed House of Representatives.

Outside, many thousands of spectators are watching from the West Lawn of the Capitol.

"I am most grateful to address this joint session of Congress in the land of the free and the home of the brave," he began his closely scrutinised speech.

He was applauded as he appealed for tolerance towards immigrants, always a touchy US political issue.

"We, the people of this continent, are not fearful of foreigners, because most of us were once foreigners," he said.

"I say this to you as the son of immigrants, knowing that so many of you are also descended from immigrants".

He also called for the abolition of capital punishment.

"I also offer encouragement to all those who are convinced that a just and necessary punishment must never exclude the dimension of hope and the goal of rehabilitation," he said.

The Pope touched on the dangers of ideological extremism, too.

The address comes at a time when Congress is a week away from another possible government shutdown in a row over abortion.

But a passing mention of the need to "protect and defend human life at every stage of its development" was Francis' only possible allusion to that issue.

Mr Boehner, a former altar boy, invited the Pope to speak after trying unsuccessfully to entice previous pontiffs to the Capitol.

Francis delighted Democrats at the White House on Wednesday when he praised President Barack Obama's efforts to fight climate change.

But at least one Republican congressman, Paul Gosar, a Catholic, said he would boycott the Pope's address to Congress because of his global warming views.

House Speaker Boehner denied the Pontiff would be stirring up an ideological hornet's nest.

"The Pope transcends all of this," he said. "He appeals to our better angels and brings us back to our daily obligations".

Catholics have come a long way in US politics since President John F Kennedy had to assure voters he wouldn't take orders from Rome.

The top House Democrat and Republican, Nancy Pelosi and John Boehner, and Vice-President Joe Biden are all Catholic, as are six of the nine Supreme Court Justices.

Later the leader of the world's 1.2 billion Roman Catholics heads to New York where he will address the United Nations on Friday.

The Pope ends his six-day visit to the US with two days in Philadelphia.

On Wednesday, Francis warned US bishops the "crimes" of the clergy sex abuse scandal must never be repeated.