"A total of 70 people have been arrested from around Kathmandu," a senior police officer said on condition of anonymity, one day before the protest.

The arrests came on the orders of the Home Ministry, the officer said.

The detainees are being held at a police training camp and various district police offices around the capital, he added.

No charges have been brought and the officer said he had "no clue" how long they would remain in detention.

Mobile phone operators said they cut off services early Thursday in accordance with a government order. Shrish Shumshere Rana, Minister for Information and Communications, confirmed the shutdown.

"The government is heightening its preparation for agitation, as the Home Ministry has said that there is a high possibility of violence during the demonstration on Friday," the minister told AFP.

Rana declined to say how long mobile telephone coverage would be cut. "We will see the situation and act accordingly."

Leaders of seven opposition parties sidelined when King Gyanendra seized power nearly a year ago called the rally to demand a restoration of democracy and to publicise their call for a boycott of municipal elections due on February 8, which they have labelled a "sham".

The rally was announced in defiance of a ban on public gatherings announced earlier in the week by the government, which also imposed a late-night curfew on the capital Kathmandu.

The government said it feared the protest would be infiltrated by Maoist rebels, who carried out a series of deadly attacks around the capital last weekend.

Maoist leader Prachanda said in a statement Wednesday the rebels would steer clear of the rally.

The European Union voiced disquiet over the arrests.

"The EU heads of mission in Kathmandu are extremely concerned about recent developments in Nepal, in particular about the arrests of democratic party leaders, members of civil society and human rights activists," the EU said a statement.

Kashinath Adhikari, secretary of the Nepal Communist Party (United Marxist Leninist), said at least 15 of his party's leaders were detained at their homes before dawn, while the Nepali Congress party said 24 of its members were picked up.

"This is an attack on the fundamental rights of the people and the royal government has tried to take away people's rights by imposing the curfew and the ban on demonstrations," Congress party secretary Shobhakar Parajuli said in a statement.

Powerful neighbour India condemned the crackdown.

"These actions of his majesty's government of Nepal are regrettable and a matter of grave concern to all those who wish to see the constitutional forces in Nepal working together to achieve peace and stability in the country," the foreign ministry said.

India had been vocal in its criticism of King Gyanendra when he sacked the elected government on February 1 last year and had joined the United States and Britain in suspending arms shipments.

New Delhi later resumed "non-lethal military aid" following assurances from the monarch that he would restore democracy.

The United Nations Office for the High Commissioner for Human Rights, meanwhile, questioned the legality of the detentions.

The arrests were made under the Public Security Act, "but this does not allow for arrests at night," said Ian Martin, the head of the office in Nepal.

Martin said he was seeking access to detainees.

A rally by the seven opposition parties in the southern town of Janakpur on January 14 attracted tens of thousands of protesters, according to eyewitnesses. Organizers hope even more will turn out in the capital on Friday.