Ahmadinejad, a close ally of President Hugo Chavez, is due to arrive on Tuesday night at the end of a Latin American tour that also took in Brazil and Bolivia. He and Chavez are likely to sign new business and industrial agreements.
"We repudiate the visit by the undesirable dictator Mahmoud Ahmadinejad with whom the current Venezuelan government has been building a supposed strategic alliance," a group of opposition parties said in a statement.
Ahmadinejad clinched a second term after a disputed June election lead to the worst unrest in Iran since the 1979 Islamic revolution and a heavy-handed clampdown on opponents.
He is now under pressure from the West to accept a U.N. plan aimed at keeping check on Iran's nuclear ambitions. The trip to Latin America helps cement ties with countries that back Iran's right to develop atomic power for peaceful purposes.
Iran is helping Venezuela map untapped uranium reserves.
Both populist and fiery anti-U.S. ideologues, Chavez and Ahmadinejad have met several times in the last few years, worrying Washington.
U.S. officials believe Venezuela helps Iran sidestep financial sanctions and say they are worried about its growing presence in the region.
Socialist Chavez broke relations with Israel this year, and accuses it of "genocide" for a military offensive in the Gaza Strip that killed hundreds of civilians, winning him wide support in the Muslim world.
His fierce speeches against Israel are taken by some of his supporters as a green light to anti-Semitism and walls in Caracas are often daubed with graffiti against Jews.
Ahmadinejad, who denies the Holocaust and has called for Israel to be wiped off the map, benefits from Venezuela's diplomatic support for its nuclear program.
"Receiving Ahmadinejad is to admit an ominous character, who represents the Dark Ages for the Iranian people," the Confederation of Israelite Associations of Venezuela said in a statement.
Iran has built houses, dairies and cars in Venezuela, which in return has promised to supply 20,000 barrels per day of gasoline — a move designed to help the Middle Eastern country dodge potential fuel sanctions.
(Reporting by Frank Jack Daniel; Editing by Cynthia Osterman)
