The Nepalese authorities say they have arrested scores of opposition leaders and activists ahead of planned pro-democracy demonstrations on Friday.
Police kept a close watch on the capital as arrests were made
All telephone services in the capital, Kathmandu, were cut off for a time, and mobile phones are still not working.
The government has also declared an all-day curfew on Friday.
Opposition parties have been planning a big demonstration in Kathmandu on Friday, despite a recent ban on rallies in the city.
The indefinite ban on all rallies follows rebel attacks last weekend.
The BBC's Charles Haviland in Kathmandu says the arrests and the cutting of phone lines appear to be aimed at foiling Friday's planned rally that was called in protest at government plans for local elections next month.
The opposition says the elections are a step to entrench the King Gyanendra's rule.
Nepalese Home Minister Kamal Thapa said that at least 100 opposition leaders and activists had been detained for security reasons.
Those detained appear to be mainly the second rung of leaders in the seven-party alliance who are opposed to King Gyanendra.
Mobile services remain cut off throughout Kathmandu
At least one very well-known human rights campaigner has also been picked up.
The authorities had asked the seven-party opposition alliance to call off Friday's rally, saying they had information that Maoist rebels were planning to infiltrate it and incite violence.
The rebels have denied any such plans.
India's foreign ministry issued a statement in Delhi calling the actions of the Nepalese government "regrettable and a matter of grave concern" to all those trying to achieve peace and stability in Nepal.
The European Union has also voiced concerns over the arrest of opposition leaders and human rights activists, and said it was urgently considering the implications of the government move.
The office of the United Nations high commissioner for human rights in Nepal said that it regretted the complete ban on demonstrations, which represented an extreme limitation on the right to freedom of peaceful assembly.
Interior Minister Kamal Thapa has warned of strong action if the parties do not call off the "agitation".
Students came out in protest in Kathmandu against the arrests
The opposition parties have been holding protests over King Gyanendra's seizure of executive powers in February last year.
They have also decided to boycott nationwide municipal elections scheduled for next month, and said they would protest peacefully against the elections.
The Maoist rebels have also vowed to disrupt the municipal polls, and have warned that they will step up their attacks against government targets in the run-up to the polls.
Earlier this month, the rebels abandoned their four-month unilateral truce, and since then there has been an upsurge in violence.
Twelve policemen were killed in Maoist attacks in Kathmandu over the weekend, prompting the government to impose a night-time curfew and a ban on all rallies.
More than 12,000 people have died in Nepal since the Maoists began their insurgency 10 years ago.
What is your reaction to the arrest of opposition activists? Will you or anybody you know be taking part in the rallies on Friday? Are you concerned about possible disruptions by Maoist rebels?
Yes I fully support the arrest of the opposition leaders because I am tired of these demonstrations which are organised 300/365 days. Only the people suffering from these demonstrations understand how seriously has it affected our daily life. I am not a fan of monarchy but these democratic leaders do no good to poor people. Time has come for someone new, someone educated, some one who would really understand the need of a common people and work for the people.
Danish Shrestha, Kathmandu
The arrest is a clear signifier of a very reactionary nature of the autocratic government. What happened to freedom of expression and the right to organize? This draconian curfew which is being increased by an hour each day is absolutely unacceptable and perhaps there will be a day-long curfew tomorrow. I will take part in the rally tomorrow even if there is a curfew. students are already planning a major demonstration. I am confident that the Maoists will keep their word, and will not disrupt tomorrow's rally.
Retika , Kathmandu, Nepal
King Gyanendra seems to have been guided by an authoritarian instinct. He should realise that he is making the situation worse. However, it is also true that whatever is happening in Kathmandu is happening for the vested interests of small groups of political and ruling elites.
Dinesh Gautam, London
As long as we can go about doing our daily jobs, I do not think most people in Kathmandu are concerned about who has been arrested. I personally do not know of anyone who will be taking part in the rallies on Friday - probably, they will mostly be hired people from outside the capital. Yes, I am concerned about possible disruptions by Maoist rebels. It is a very good opportunity for them to create trouble - perhaps burn and smash everything in sight - as it usually happens in such demonstrations.
Pramesh P, Kathmandu