Pope Francis has denounced as "slave labour" the conditions of workers caught in a deadly building collapse in Bangladesh last week.
More than 400 people are confirmed to have died in the collapse of the Rana Plaza building near the capital, Dhaka.
It housed several clothing factories, some supplying Western retailers.
At May Day parades in Dhaka, marchers demanded the death penalty for the building's owner and better conditions for workers.
The Pope said he had been shocked by reports that some of the labourers had been paid just 38 euros ($50) a month.
"Today in the world this slavery is being committed against something beautiful that God has given us - the capacity to create, to work, to have dignity," the Pope said at a private Mass.
"Not paying a fair wage, not giving a job because you are only looking at balance sheets, only looking to make a profit, that goes against God," he was quoted as saying by Vatican radio.
At least 410 people are confirmed to have died and more than 140 are missing following the collapse of the eight-storey building a week ago, police and army officials said. Some 2,500 people were injured.
It was the country's worst industrial disaster.
More than 30 of those killed, whose bodies have not been identified, were buried in a mass funeral on Wednesday.
In Dhaka, an estimated 20,000 people took part in the main May Day march, while separate demonstrations were held in other parts of the capital and elsewhere.
"I want the death penalty for the owner of the building," said one marcher, 18-year-old garment factory worker Mongidul Islam Rana.
"We want regular salaries, raises and absolutely we want better safety in our factories."
Others in Dhaka held banners with the words: "Hang the killers, Hang the factory owners."
Speaking at a rally in the industrial township of Narayanganj, the leader of Bangladesh's main opposition party, Khaleda Zia, alleged that the government was hiding the real casualty figures from the building collapse.
She also claimed that if the army had been given control of the rescue operation earlier, more lives could have been saved.
The European Union has said it is considering "appropriate action" to encourage improvements in working conditions in Bangladeshi factories.
It said its actions might include the use of its trade preference system, which gives Bangladesh duty- and quota-free access to EU markets.
Bangladesh's garment industry makes up almost 80% of the country's annual exports and provides employment to about four million people.
However, it has faced criticism over low pay and limited rights given to workers, and for the often dangerous working conditions in factories.
Both Primark, which has a large presence in the UK, and Canadian company Loblaw had clothing made in the Rana Plaza, and have said they will offer aid to victims and their families.
Rana Plaza owner Mohammed Sohel Rana, a local leader of the youth wing of the ruling Awami League party, is in police custody.
A total of eight people, including factory owners and engineers, have been arrested for alleged negligence.