Electoral reform group Bersih said its website has come under persistent denial-of-service (DDOS) attack since September 13.
"Our website is now restored and we are making all efforts to thwart the attacks," it said in a statement today.
A DDOS attack utilises machines to unnecessarily load a site to an extent that it overloads.
"Our server received over 460 million requests in the last few days. The top five origins of this attack are from Indonesia, Russia, Thailand, China and India, but it is impossible to determine the originator of this attack.
"What is clear is that this is a concerted attack and likely a well-funded effort to bring down our website," it said.
Bersih said it does not know who is perpetrating the attacks.
The last statement on its website at www.bersih.org prior to the cyber attack was on Hong Kong.
In the statement, Bersih clarified its position on the Hong Kong protests.
The electoral reform group had called for the release of arrested pro-democracy activists in Hong Kong. It also urged the Hong Kong government to heed the demands of protesters.
Bersih, which had organised major peaceful rallies in Malaysia, said it supported the right to peaceful assembly, but expressed concern about violence perpetrated by police and radical protesters in Hong Kong.
Bersih said it will continue to issue statements on its website and will turn to Facebook as an alternative if its website is inaccessible.
"Bersih do not know who would want to target us, but such illegal acts will not deter our commitment to speak up on behalf of democracy and freedom, both in this country and abroad.
"We condemn in the strongest terms such acts of intimidation and attempts to stifle freedom," it said.