Opposition leaders in Zimbabwe have urged voters to boycott legislative elections on Saturday and instead prepare to take to the streets and challenge President Robert Mugabe after 25 years of increasingly authoritarian rule.

Although it appeared unlikely that street protests would begin soon, analysts predicted an extremely low turnout for the elections, which come at a time of increasing economic problems, public discontent and tensions between the Mugabe government and the United States.

Opposition leaders said a very low turnout would signal the end of a six-year period in which they attempted to bring political change through elections. During that time, however, there was recurring evidence that Mugabe was manipulating the results.

Mugabe, in pre-election campaigning, has called for a heavy turnout to demonstrate the vibrancy of Zimbabwean democracy.

U.S. Ambassador Christopher W. Dell, who was threatened with expulsion this month after publicly criticizing the government, was quoted Thursday in Zimbabwe's weekly Financial Gazette accusing Mugabe of "Nazi" tactics.

The front-page story, from a Nov. 8 interview, quoted Dell as saying, "It is interesting that the government is using tactics used in Nazi Germany, where you accuse another of doing exactly what you are doing as a distraction." U.S. officials, speaking on condition of anonymity, did not dispute the quote.

In recent years, Mugabe has closed newspapers, outlawed many forms of political freedom and overseen the collapse of the Zimbabwean economy. In a drive to clean up crowded urban communities, he has also demolished poor districts and driven residents out, drawing international criticism.

The nominal focus of Saturday's vote is the election of members to the Senate, a new upper chamber of parliament that opposition figures have dismissed as little more than a jobs program for regime cronies. It will have few official duties and will not be able to block legislation passed by the lower chamber.

But the call for an election boycott by opposition leader Morgan Tsvangirai has turned the vote into a referendum on the usefulness of elections themselves. The elections have also been opposed by influential groups that include the National Constitutional Assembly, Women of Zimbabwe Arise and the Zimbabwe Congress of Trade Unions.

"The elections have brought nothing by way of meaningful change to Zimbabwe," said Tsvangirai's spokesman, William Bango, speaking from Harare, the capital. "Mr. Tsvangirai is organizing Zimbabweans to confront the dictatorship. . . . Only through people power can Zimbabweans change their lives."

Tsvangirai has long endured criticism from other opposition figures and outside analysts for not challenging Mugabe more aggressively, especially after national elections in March and in 2002. Protests are illegal in Zimbabwe without explicit police permission, and Tsvangirai has said he was reluctant to put his supporters in harm's way.

Now, his call for new tactics, including the boycott, has met fierce resistance from within his own party, the Movement for Democratic Change. A dissident wing of the MDC, dominated by the minority Ndebele ethnic group from southern Zimbabwe, has refused to take part in the boycott. It also has accused Tsvangirai of using heavy-handed tactics and undermining party unity.

The dissident wing has fielded 26 candidates in the 50 contested races. An additional 16 seats will be appointed, making for a total of 66 in the Senate. Analysts agreed that a strong showing among the dissident candidates could weaken Tsvangirai's leadership and his drive to move toward stronger tactics against Mugabe.

A poor showing by the dissidents, coupled with a weak turnout nationwide, could by contrast strengthen Tsvangirai. It could give him momentum heading into the party's national congress in February, which will seek to settle the direction of the opposition movement.

John M. W. Makumbe, a Zimbabwean political analyst, predicted that the turnout Saturday will be less than 20 percent, far below the previous record low of 31 percent in 1996. That result, he said, would lead to a major shift in the Movement for Democratic Change, which was founded in 1999 by union leaders, human rights activists and civil society groups.

"It will stop taking for granted that you can remove a dictator from power through democratic means," said Makumbe, speaking from Harare. "Make the dictator struggle to keep peace."

Faced with such complex political dynamics, Mugabe has been campaigning almost entirely in the southern region, where the opposition party and its dissident wing are strongest.

The leading state-owned newspaper, the Herald, on Friday predicted a turnout of more than 3.2 million in a country of 12 million, well above what most analysts expect in an election that has produced few rallies, campaign posters or advertisements on radio and television.

Lovemore Madhuku, chairman of the National Constitutional Assembly, predicted that the vast majority of opposition supporters would not vote.

"They are getting fed up," he said in a telephone interview from Harare. "You will see."