A U.S. official on Friday denied claims by President Hugo Chavez that Washington masterminded an opposition boycott of this weekend's Venezuelan elections and was trying to foment an overthrow of his leftist government.

Chavez accused President Bush late Thursday of being behind the withdrawal of Venezuela's major opposition parties from Sunday's congressional elections, saying he had proof the CIA was "encouraging this new conspiracy." He provided no evidence.

"The decisions made by the political parties were their decisions alone," Brian Penn, a U.S. Embassy spokesman, said. "We are simply not responsible for everything that goes on in Venezuela."

Penn said the Venezuelan government had made "dozens" of baseless accusations against the United States and insisted: "We support the democratic process."

Opposition parties have claimed fair elections cannot be held because conditions are biased toward pro-Chavez candidates. Chavez accuses them of pulling out on Bush's instructions and because they realize they will suffer big losses. The opposition candidates have trailed in public opinion polls.

The boycott was the latest in a series of opposition moves Chavez has attributed to the U.S. government, including a short-lived coup against him in 2002, a crippling oil strike in late 2002 to 2003 and a failed recall referendum last year.

Warning that the latest alleged conspiracy could lead to a violent effort to oust him, Chavez said he put the military high command on alert and called for "all Venezuelans to mobilize permanently across the country." But he said he wasn't overly worried about being driven from power.

"Mr. Bush, I'm going to make another bet with you. I've bet you a dollar to see who lasts longer -- you in the White House or me here in Miraflores," Chavez said Thursday, referring to Venezuela's presidential residence.

The opposition boycott clears the way for candidates aligned with Chavez to expand their dominance of congress.

Pro-Chavez candidates are aiming to win a two-thirds majority -- up from their current 52 percent -- in the 165-seat National Assembly. That would allow them to rewrite portions of the constitution and push back term limits for the presidency and other offices.

Opposition parties have accused the national electoral council of a pro-Chavez bias and expressed concerns that a computerized voting system could compromise confidentiality. The parties that have pulled out include Democratic Action, the Social Christian party and Justice First. Jorge Rodriguez, the National Elections Council chief, said Friday that only 322 candidates of about 5,500 had formally withdrawn.

President Hugo Chavez called election boycott proof of a new U.S. conspiracy.