Ali Kerimli of the main Azadliq opposition bloc said the opposition would not be silent if "massive falsifications" take place.
Also, the New York-based Human Rights Watch (HRW) today questioned the timing and significance of Azerbaijan's new election rules.
HRW said in a statement that the new measures are "too little, too late."
Azerbaijan's parliament recently authorized NGOs that receive more than 30 percent of their funding from abroad to monitor the 6 November legislative polls. Also, election authorities pledged to mark voters' fingers with ink and to use accurate voter lists and reliable voter identification documents.
The United States welcomed the decisions. It said that, if implemented, they would help reduce the risk of fraud.
But HRW says restrictions on freedom of assembly, arrests of opposition activists, and pro-government bias in the media during the election campaign "have already fundamentally tainted the integrity of the vote."
Azerbaijan's presidential administration official Ali Hasanov this week accused HRW of "bias."
RFE/RL's complete coverage of the November 6, 2005, legislative elections in Azerbaijan and their aftermath.
Timeline: How The Elections Have Unfolded
A photo gallery of the Azerbaijan opposition protests in Baku on November 9, 2005.