MANAGUA, NICARAGUA, FEB. 16 -- The issuance this week of a new currency, which has brought a new level of chaos to commerce in Managua, could also cause complications for thousands of antigovernment rebels loaded down with the old money.

Nicaraguan Vice President Sergio Ramirez said today on a national radio station that the planning for the currency change took place in secret to surprise "enemies of the revolution."

Ramirez said the government was closing Nicaragua's borders today and Wednesday to prevent rebels and their supporters from crossing into the country to change money.

"We think there is a very large quantity of old money outside the country, and now that money is completely worthless," Ramirez said.

The currency change was announced Sunday by President Daniel Ortega, who said 60,000 workers prepared in secret for the issuance of the new money.

The rebels, known as contras, receive money from U.S. government-sponsored flights, which drop supplies at prearranged sites. The rebels use this money to buy food and other goods in the Nicaraguan countryside. These supply flights are to stop Feb. 29 because of congressional defeat Feb. 3 of a Reagan administration aid request.

Because of the 1,500 percent inflation, the figure acknowledged by Ortega, the rebels, and others, have been forced to carry around bags full of bills -- which, as of Thursday, are to be valueless.

Nicaraguans began lining up yesterday to change the old currency, known as the cordoba, for the new money, which will be known as the new cordoba and will be worth 1,000 of the old currency. The cordoba was introduced in 1919. Wednesday is the last day to change it for new.

Marta Sacasa, a spokeswoman for the Nicaraguan Resistance, denied in a telephone interview from Miami that the rebels had been surprised by the currency change. "We've been expecting this for some time," she said.

Sacasa denied the change will harm the rebels. "It will help us, because fighters won't have to carry around bags of money," she said.

Sacasa would not say how the Nicaraguan Resistance plans to get the new currency to its fighters. "Everybody knows it's easy to get cordobas in Nicaragua if you have dollars," she said.

She said rebel fighters will live off of the generosity of Nicaraguan peasants until the situation normalizes. "In a week, we will be able to get money to the combatants," she said.

The government is mounting a massive operation to increase consumer confidence in the new currency. Most radio stations here are broadcasting news and information about the changes 24 hours a day. Government economists have hosted several call-in radio talk shows to answer questions.

Nevertheless, many Nicaraguans have not adjusted. Many buyers say vendors refuse to accept the new currency, while taxi drivers turn down the old currency.

Basic goods such as rice, meat and milk were not available in stores today. Telephone operators of the government-run telephone system prevented callers from charging calls in Nicaragua, saying they were unsure how to charge the calls with the new currency.

"Everybody says they are waiting to see whether the new money is really worth anything," said Maria Andino, a shopper at the sprawling Eastern Market here today.

Alejandro Martinez Cuenca, minister of foreign commerce, said in a press conference yesterday that the smooth functioning of the new system will depend on consumer confidence in the new currency.

La Prensa, the antigovernment newspaper, reported yesterday that the black-market exchange rate for dollars increased 10-fold during the day. Several businessmen said they have been changing the dollars for six or seven times the official exchange rate.