CARACAS, Venezuela (AP) - Tens of thousands of President Hugo Chavez's supporters filled the streets Friday vowing to win a vote on constitutional changes that would let the firebrand leftist seek re-election for years to come. Opponents called for close monitoring of results in what they expect could be a tight contest.

Government supporters, who filled an avenue waiting to hear Chavez speak, said it was essential to do away with term limits so that he could keep leading Venezuela beyond 2012.

"Chavez has become a father for us," said Xiomi Diaz, a 34-year-old farmer. "He's a father of the poor." She and others traveled from across the country in hundreds of buses.

The pro-Chavez rally came a day after more than 100,000 opposition supporters filled the same avenue promising to defeat revisions that would also extend presidential terms from six to seven years, create new forms of communal property and expand Chavez's powers to reshape Venezuela as a socialist state.

Chavez denies he is trying to amass power, saying the changes are necessary to give the people a greater voice in government and to move toward a socialist system.

Human Rights Watch warned the reforms would threaten fundamental rights, citing one revision allowing the president to declare indefinite states of emergency during which the government could detain citizens without charge and censor the media.

"These amendments would enable President Chavez to suspend basic rights indefinitely by maintaining a perpetual state of emergency," said Jose Miguel Vivanco of the New York-based group.

In Washington, State Department spokesman Sean McCormack said the United States hopes the referendum will be "a free and fair contest in which the will and desire of the Venezuelan people is reflected."

Chavez's opponents have questioned the National Electoral Council's impartiality, especially after Chavez named its former chief, Jorge Rodriguez, as his vice president in January. But in contrast to past elections, when the opposition has boycotted votes or been split on whether to participate, this time many opposition leaders are emboldened and urging voters to turn out in large numbers.

The government and the opposition both cite polls suggesting their side is in the lead, raising tensions ahead of the referendum.

University students have led protests and occasionally clashed with police and Chavista groups. One man was shot dead Monday while trying to get through a road blocked by protesters.

The opposition also has been heartened by some recent defections from Chavez's movement, including former Defense Minister Gen. Raul Baduel. Even Chavez's ex-wife, Marisabel Rodriguez, has urged Venezuelans to vote "no."

About 100 electoral observers from 39 countries in Latin America, Europe and the United States are on hand, plus hundreds of Venezuelan observers, the National Electoral Council said.

Yet, absent this time are the Organization of American States and the European Union, which have monitored past votes. The short notice before the vote - the reforms were approved by the solidly pro-Chavez National Assembly just one month before - has prevented both institutions from sending delegations that normally take six months to prepare.

Among those attending are observers from the National Lawyers Guild and the National Association for the Advancement of Colored People.

Chavez's opponents have been suspicious of electronic voting machines made by Boca Raton, Fla.-based Smartmatic Inc., which is primarily owned by three Venezuelans. But Luis Enrique Lander of the Venezuelan vote-monitoring group Ojo Electoral said his team is satisfied with vote preparations and safeguards.

As in past elections, Venezuelans will get paper receipts that verify their choices were properly recorded, and must deposit them into boxes before leaving the polls. Lander said that is a key safeguard, in addition to an audit of the paper slips at 54 percent of polling stations.

Chavez, who was first elected in 1998, has dismissed some polls suggesting a close race. He was handily re-elected last year with 63 percent of the vote. This time, he has warned his opponents could try to deny the results and launch violent protests.

Chavez's close friend and ally Fidel Castro said Friday he has warned the Venezuelan leader to watch out for assassination attempts, telling him to avoid open-top vehicles that could be targeted by snipers.

"On Chavez's recent visit last Nov. 21, I seriously discussed with him the risks of assassination as he is constantly out in the open in convertible vehicles," Castro wrote in an essay published in Cuba's official newspapers.