Pope Francis will meet Sunni Islam's leading authority on Friday during a visit to Bahrain aimed at increasing dialogue but marked by accusations of rights abuses in the Gulf state.

The pontiff landed in the tiny island kingdom on Thursday afternoon, delivering a speech critical of the use of the death penalty and urging nations to respect human rights and provide better conditions for workers.

Rights groups had previously called on the pope to speak out about alleged abuses in the Sunni-led monarchy, which is home to a significant Shiite population.

Speaking in front of dignitaries including his host, King Hamad bin Isa Al-Khalifa, the leader of the world's 1.3 billion Catholics said religious freedom should be "complete and not limited to freedom of worship".

On Friday, the 85-year-old pope, who uses a wheelchair because of chronic knee problems, plans to meet Sheikh Ahmed al-Tayeb, the grand imam of Cairo's prestigious Al-Azhar mosque and centre of Sunni learning.

The pontiff's 39th international trip since taking office comes three years after he signed a Muslim-Christian manifesto for peace in the United Arab Emirates during the first papal visit to the Gulf region, where Islam was born.

The Argentine has placed inter-faith dialogue at the heart of his papacy, visiting other Muslim-majority countries such as Egypt, Turkey and Iraq.

The visit to Bahrain, where he will also attend a forum with the Abu Dhabi-based Muslim Council of Elders, is no exception.

But ahead of his trip, rights groups and the families of Bahraini prisoners on death row pressed him to address alleged violations.

- Labour 'dehumanising' -
In the first speech of his visit, at the sumptuous Sakhir Royal Palace, he said it was vital that "fundamental human rights are not violated but promoted".

"I think in the first place of the right to life, of the need to guarantee that right always, including for those being punished, whose lives should not be taken," he said.

Bahrain has executed six people since 2017, when it carried out its first execution in seven years. Some of the condemned were convicted following a 2011 uprising put down with Saudi military support.

A government spokesman rejected allegations of rights violations, saying Bahrain "does not tolerate discrimination" or prosecute anyone for their religious or political beliefs.

Speaking less than three weeks from the World Cup in neighbouring Qatar, which has faced fierce scrutiny over its treatment of migrant workers, the pope also demanded that "working conditions everywhere are safe and dignified".

"Much labour is in fact dehumanising," he said. "This does not only entail a grave risk of social instability, but constitutes a threat to human dignity."

Pope Francis, after being lowered from the plane in his wheelchair on an electric platform, received an enthusiastic welcome on Thursday.

Uniformed guards on horseback, along with Vatican and Bahrain flags, lined the route of his short journey to the gleaming, white-marble palace, where he was greeted by cheering children.

As well as attending the forum and meeting the grand imam, the pope will also hold prayers in the Our Lady of Arabia Cathedral, the biggest Catholic church in the Arabian peninsula, followed by an open-air mass on Saturday.