The National League for Democracy in Burma says the health of its jailed leader Aung San Suu Kyi is a cause for "grave concern."
The NLD says Ms Suu Kyi, 63, has not been able to sleep at night because of continuing cramps in her legs and needs urgent medical attention.
She is on trial for violating the terms of her house arrest. She denies the charges and faces five years in prison if found guilty.
One of her lawyers, Nyan Win, said the court has decided to delay final arguments in her case until 5 June. They had been expected to take place on Monday.
Ms Suu Kyi is widely expected to be convicted. Observers believe that Burma's military leaders will seize on the incident to keep her behind bars during what they say will be multi-party elections in 2010.
Ms Suu Kyi is being held in the Insein prison in Rangoon. She has suffered from ill-health in the past.
Shortly before her arrest on 14 May, she was treated for dehydration and low blood pressure.
The NLD said she was "in desperate need of proper medical treatment" and it was "very much concerned about her health."
The BBC reports Ms Suu Kyi has been under house arrest and banned from seeing all but a small group of people for 13 of the past 19 years.
She was arrested on 14 May after an American man swam across the lake to reach her compound, spending two nights there.