JOHANNESBURG - Thieves posing as eager art students with their teacher stole more than $2 million worth of paintings from a museum in South Africa's capital in a daring armed robbery, authorities said Monday.  

The theft Sunday at the Pretoria Art Museum saw robbers calmly pay $2.25 apiece for tickets and ask a curator to show them specific paintings at the gallery before they pulled out pistols and forced all others to the ground, officials said. They tied up the curator and others before collecting the paintings they previously asked about, official said.  

The robbers favored oil paintings in their theft, grabbing a 1931 painting by famous South African artist Irma Stern of brightly colored sailboats waiting against a pier, city spokesman Pieter de Necker said. Other works stolen included a gouache drawing of an eland and bird by South African landscape artist J.H. Pierneef, a pastel-toned street scene by Gerard Sekoto, a thick-stroked oil painting of a chief by Hugo Naude and a picture of a cat near a vase full of petunias by Maggie Laubser.

The robbers, though apparently having done their homework, left behind another oil painting by Stern showing two musicians because they were not able to fit the painting inside their getaway car, a silver sedan, de Necker said. The thieves left as private security guards at the museum drew close to them, he said.

The museum closed Monday for the week and removed its most valuable remaining possessions for safekeeping, the city spokesman said. Authorities say they now plan to increase security to prevent thefts there. However, the video surveillance cameras at the museum had stopped working on Thursday, de Necker said.

South African authorities had been alerted in case the thieves tried to take the art work outside of the country, said Lt. Col. Katlego Mogale, a spokeswoman for the South African Police Service. She declined to offer any other specifics about the crime, saying police still were piecing together what actually happened at the museum.

"The investigation is continuing," the lieutenant colonel said. "Every measure is being put in place."

Violent crime and murders remain common in South Africa, but high-profile art thefts are rare. In February 2011, thieves stole four small, limited-edition prints by South African artist William Kentridge from a gallery in Johannesburg. Thieves also have targeted bronze statutes in other South African museums, with authorities believing they are actually simply sold for their scrap metal value.

De Necker said he and others believe the thieves were commissioned to go after those specific pieces because of their behavior at the museum.

"We're very, very surprised. It is very uncommon," the spokesman said. "We have realized also that over the last few years ... the overseas market has grown into wanting South African art."

Art theft is the third most lucrative crime in the world, after drugs and illicit arms sales, according to Interpol and the FBI. However, actually selling famous works remains difficult for criminals either locally where the theft happened or abroad, authorities say. Despite the challenges, estimates suggest there are billions of dollars made in stolen art sales annually across the world.