CARACAS, Jan 17 (Reuters) -

Venezuela's Chavez urges tear gas against protests

Venezuelans will vote next month on a proposed change to the constitution that would allow Chavez, a foe of the United States, to seek re-election when his term ends in four years.

In 2007, voters rejected a package of political reforms that would have allowed him to run again for the top office.

Small groups of students in gas masks and wielding plastic shields protested the proposal this week. They threw stones at police, blocked a highway and were accused of setting fire to a national park. Chavez said police on his orders used tear gas to disperse the protest.

Chavez said the protest was part of a U.S.-backed plan to destabilize the oil-exporting nation ahead of the referendum.

On Saturday, he told security forces to use gas and water cannons at the first sign of trouble.

"Interior Ministry, spray them with gas and dissolve any disturbance. We cannot begin showing weakness as a government," Chavez said during a campaign meeting at a historic Venezuelan battleground.

Popular for raising the living standards of poor Venezuelans, Chavez has governed for a decade but says he needs 10 more years to extend social reforms in one of the United States' main oil suppliers.

Polls last month showed the new proposal had about 40 percent support, although pollsters expect that to rise.

Chavez frequently lashes out at opponents and the United States during election campaigns. He tries to motivate supporters with the idea enemies are planning his overthrow.

On Saturday, the leftist leader said U.S. President-elect Barack Obama encouraged Venezuela's opposition to remove him, saying Obama called him an obstacle to progress in Latin America.

Last week, Chavez threatened to expel a U.S. diplomat he accused of meeting with Venezuelan opposition leaders in Puerto Rico. The U.S. Embassy denies the meeting.

Relations with the United States have worsened since a brief 2002 coup against Chavez that was initially welcomed by Washington. In September, Chavez expelled the U.S. ambassador to Venezuela. (Editing by Peter Cooney)