Three former ministers of King Gyanendra have been arrested on suspicion of conspiring to encourage recent violence in southern Nepal.
Kamal Thapa being taken away in a police vehicle
Kamal Thapa, Badri Prasad Mandal and Salim Miya Ansari were detained by police between Monday night and Tuesday morning, officials said.
Meanwhile Nepal's second biggest city, Biratnagar, has been put under curfew.
The unrest in Nepal's impoverished southern Terai region began earlier this month and spread widely.
The Madheshi people of the area say they have been left out of government development and policy-making decisions. At least eight people have been killed in the unrest.
Reports from Kathmandu suggest the government and former Maoist rebels have agreed a plan to give the Madheshis greater political representation.
Former Deputy Prime Minister Mandal was arrested at his residence in the eastern Terai town of Biratnagar on Monday night, said officials.
The Madheshi protests spread rapidly
Mr Thapa, the former royally-appointed home minister, was also detained late on Monday, at his home in the capital, Kathmandu.
He and former Forest and Soil Conservation Minister Salim Miya Ansari, who was held early on Tuesday, have been kept in a special detention centre in Kathmandu.
Home Minister Krishna Prasad Sitaula and senior police officials say the arrests follow "credible intelligence inputs which suggest their involvement in the continuing violence in the Terai region".
Several other former ministers and officials are also under surveillance.
The south-eastern city of Biratnagar was placed under an indefinite curfew on Tuesday after the death of a demonstrator there during clashes with police.
The BBC's Charles Haviland in Kathmandu says that the sudden outbreak of violence in Biratnagar will be worrying for the government as unrest there had appeared to be on the wane.
Several other cities a long way to the west, the scene of much recent violence, have remained under daylight curfew as Madheshis held protests and blockaded roads demanding better representation in government.
Most of those who have died were killed in police action against them.
But the Madheshi People's Rights Forum, which says it believes in peaceful protest, is increasingly being accused of using violent tactics.
The Federation of Nepalese Journalists says more than 12 reporters have fled from the southern plains after being threatened by forum activists, who have also vandalised media offices.
Other journalists have abandoned their workplaces after receiving death threats.
The violence in southern Nepal comes after Maoist rebels declared an end to their decade-long civil war and joined an interim parliament.
In April last year, King Gyanendra was forced to end his direct rule in the Himalayan country after weeks of mass political protests.