Afghanistan's main opposition on Wednesday criticized President Hamid Karzai's removal of foreign observers from a U.N.-backed electoral watchdog as autocratic and urged international pressure to ensure impartial elections.

The five-member Electoral Complaints Commission (ECC), tasked with reviewing voting fraud, previously had three foreign members appointed by the United Nations, and last year nullified a third of Karzai's votes in the presidential poll as fraudulent.

Karzai's decision on Tuesday could raise doubts over the panel's independence and set off new political tensions as NATO presses ahead with a major offensive against the Taliban.

It could also put him in new conflict with Western donors who have said they will not fund September 18 parliamentary elections without electoral reforms.

The main opposition National Front urged donor nations, legislators and other political parties to develop a mechanism to guarantee transparent elections.

This shows that Mr. Karzai is using one of his autocratic methods by not consulting with parliament, civil society and political parties, National Front spokesman Fazel Sangcharaki said.

Karzai's most important backer, the United States, did not criticize the move but NATO coalition member Canada expressed concern it could weaken the watchdog and imperil the credibility of the parliamentary elections.

A strong and independent ECC is vital for the future of a democratic Afghanistan, and any efforts to weaken this body are disturbing, Canadian Foreign Minister Lawrence Cannon said in a statement.

This is a blow to the process of universal, fair and free elections, Afghan opposition lawmaker Fawziya Koufi said. We knew that he would do this even before we went on leave. Karzai plays his plots this way.

Free and fair elections are part of a Western strategy to return the nation to stability, while a NATO-led military operation battles a renewed Taliban insurgency with the aim of returning all of Afghanistan to the Karzai-led government.

On Tuesday, Karzai's spokesman, Siamak Herawi, said the Afghan government had long wanted to Afghanise the electoral process. He said parliament could not overturn the law, since Karzai signed it into effect when the legislature was in recess.

Once a darling of the West, Karzai has been under fire from Western states over poor governance and for failing to clamp down on corruption and the illegal drugs industry, both of which are seen to be feeding the Taliban-led insurgency.

Opponents criticized the national elections body for failing to halt last year's fraud, which the ECC documented before throwing out nearly a million votes. That left Karzai faced with a runoff until his opponent pulled out.

Karzai has secured the support of regional power brokers belonging to various ethnic groups and won the presidential poll last year by giving them positions in the government, analysts say.

Donor nations provided security and more than $230 million dollars for the poll last year. The United Nations is holding tens of millions of dollars earmarked for this year's vote, with diplomats saying they will not release the money without reforms.

(Editing by Michael Georgy and Bryson Hull)