(Adds PDVSA confirms oil storage tank spill)

By Marcely Guanipa

MARACAY, Venezuela, Oct 31 (Reuters) - Venezuela's state oil company PDVSA said on Tuesday that a storage tank at its Amuay refinery had overflown because of heavy rains and that a cleanup had begun to avoid the stain spreading further in the surrounding bay.

Reuters reported earlier on Tuesday that a slop tank at Amuay had spilled over, polluting the bay and staining tankers, according to a union leader and two refinery workers.

Union leader and government critic Ivan Freites said it was hard to quantify the size of the spill, but that it was significant and could be around 200,000 barrels of products, including gasoline and gasoil.

PDVSA attributed the spill to heavy rains and did not provide details on its size.

Critics of leftist President Nicolas Maduro’s administration say lack of investments and lax environmental safeguards have led to an increase in polluting oil spills in the oil-rich Maracaibo Lake and along Venezuela's Caribbean coast.

Maduro’s government says a right-wing business elite is out to smear PDVSA and insists the company upholds strict environmental norms.

The Paraguana Refining Center, home to the Amuay and Cardon refineries in Venezuela’s northwest, has repeatedly reduced output in recent months, contributing to intermittent fuel shortages in the South American OPEC member.

The center is down to operating at about a third of its 955,000-barrel-per-day capacity, according to Freites and documents from PDVSA earlier this month. (Reporting by Marcely Guanipa; writing by Alexandra Ulmer; editing by Susan Thomas and Grant McCool)