The Maoists would support the Opposition groups ``as much as possible,'' rebel leader Prachanda said in a statement sent to news organisations. It was not clear if he was offering armed support.
The political parties did not immediately respond to the offer, but have said in the past they believe in peaceful protests against the power grab rather than violence and killings tactics frequently used by the rebels.
Nepal's top seven political parties have demanded that Parliament be reinstated and an interim government be set up to draft a new constitution. They said the Communist rebellion should be ended through dialogue with the guerillas.
``Our party has decided to support as much as possible the movement taken by the political parties which we consider as a positive move,'' said the statement from Mr. Prachanda, whose real name is Pushpa Kamal Dahal.
Nepal's political parties usually keep their distance from the guerillas, whose insurgency began in 1996 and has killed more than 11,500 persons.
Two Nepal Communist leaders freed
King Gyanendra has refused all the parties' proposals since he took absolute power on February 1 and declared a state of emergency a move he said was necessary to quell the insurgency. The state of emergency has since been lifted, but hundreds of Opposition members are still detained and curbs on media and other freedoms remain.
The rebels, who claim to be inspired by Chinese revolutionary Mao Zedong, have stepped up violence since the King's power grab, closed highways and called general strikes.
``We consider the political parties' refusal to form an alliance with us in the past as their weak point,'' said Prachanda.
Family members and party officials said several of the hundreds of politicians and activists detained were in poor health and that some have been denied proper medical services.
``Relatives have been turned away, and almost all of the detainees have been denied access to lawyers,'' said Gopalman Shrestha, a leader of the Nepali Congress Democratic party.
Shashi Shretha said her detained husband Lilamani Pokhrel, who leads a minor Communist party, ``has lost feeling in some parts of his body and desperately needs to be taken to specialists.''
Pokhrel has kidney and bladder stones, which doctors have said should be immediately removed, his wife said.