Swaziland police arrested an opposition leader in a crackdown denied by authorities and broke up an international trade unionist meeting on Friday, said South Africa's main labour union grouping.

Workers' federation Cosatu said Mario Masuku, the leader of Swaziland opposition party Pudemo, was detained on Friday en route to a protest against King Mswati III's government just two weeks before elections in the absolute monarchy.

"The Pudemo president was arrested on his way to the border blockade near Oshoek," in northeast Swaziland, said Cosatu's international secretary Bongani Masuku.

But Swazi police spokeswoman Wendy Hleta told AFP in the capital Mbabane that the claim was "lies."

Cosatu representative Jay Naidoo said an international group investigating workers' rights was detained for questioning Thursday before police broke up a meeting in Swaziland's second city of Manzini on Friday.

The International Trade Union Confederation, of which Cosatu is a member, planned to interview workers and hold talks in Swaziland this week.

But on Thursday "we were followed by police. We were stopped at a road block," Naidoo, a former South African cabinet minister under Nelson Mandela, told AFP.

The group was questioned for 30 minutes and held "cordial" discussions with around 25 police officers, he said.

"This morning they approached us at the hotel and said the meeting was banned," Naidoo said after flying back to South Africa.

He corrected Cosatu's earlier claims that the unionists had been deported, saying they left "voluntarily," but still denounced the government's stifling of the protest.

"The security establishment seems to be the one running the country," he said.

Africa's last absolute monarchy, which on Friday celebrated independence 45 years ago from Britain, votes in parliamentary elections on September 20.

But critics say the polls are a sham, since Mswati controls parliament and political parties have been banned for decades in what he termed last week a "monarchial democracy."

Labour groups and the Swazi diaspora are campaigning for democratic reforms in the tiny mountain kingdom, which is surrounded by South Africa and Mozambique.

Cosatu and South Africa's ruling party the ANC support the demands for reform, though Pretoria has been less openly critical against the kingdom.

In a report released Friday, Britain-based think tank Chatham House urged political and economic change in the kingdom, which it labelled "Southern Africa's Forgotten Crisis."

A bloated civil service and an expensive royal household have crippled the impoverished kingdom's economy.

"The single greatest risk for the next government is to believe that the country can continue to be ruled effectively without introducing reforms," the report said.

Mswati's reportedly has a personal fortune of $200 million (152 euros), while 70 percent of his subjects live below the poverty line, according to the UN.

A Swazi newspaper editor and former adviser to Mswati who fled the kingdom in February 2012 said everything revolved around the king in the "monster that is Swaziland."

"Fear is what the king uses to rule Swaziland. There is no freedom of expression in Swaziland," said Musa Ndlangamandla, who was sacked as editor of the state-controlled Swazi Observer after he published interviews with leaders of banned pro-democracy groups.

But Chatham's Africa research director Alex Vines said many reform-minded activists and politicians would take part in the upcoming polls, which could bring about change.

"After 20 September there will be more MPs in Swaziland that have a reformist vision," he said at the release of the report in Johannesburg.

"The king might have to consider taking some of those into government."