MCI Communications Corp. began competition with international courier, post and Telex services today with the opening here of a distribution center for electronically transmitted documents.

The distribution center, which MCI will operate in cooperation with the state-run Belgian postal service, is designed to allow faster international delivery of documents at prices up to 50 percent cheaper than those of existing courier services, MCI officials said.

The documents, sent instantly from MCI facilities in the United States, will be reproduced in Brussels with laser printers, then sent to their designation either through postal services or by courier.

The officials said next-day delivery, following transmission from the United States, was possible for many European cities.

MCI officials said they estimate the size of the market for the service to be $2 billion a year, with a growth potential of 30 percent to 50 percent. They said they see the main competitors as courier and postal services and, to a much lesser degree, Telex services.

MCI opened its bid for a share of the international telecommunications market last fall, beginning cut-rate telephone service from the United States to several European and South American countries. That move brought MCI into competition with American Telegraph & Telephone Co. for part of the $5 billion-a-year international telephone market.

MCI officials praised the Belgian government for what they termed a liberal attitude toward cooperation with an American company in the telecommunications field. Orville Wright, president of MCI, said at a press conference that, in addition to the Belgian government's favorable attitude, the fact that a number of American companies have their European headquarters in the city was a factor in the choice of Brussels as the location of the distribution center.

Wright said he expected that most of the users of the service will be U.S. business executives who need rapid transmission of letters, contracts and memoranda.

The Brussels center will serve as the distribution point for documents to Europe, Africa, the Middle East and Asia. Wright said MCI is holding exploratory talks with several Asian governments about setting up a distribution center in their countries to provide for faster service from the United States to Asia. Documents for South and Central America are distributed from Miami.

MCI officials said that although equipment in Brussels could be used to send documents to the United States, they faced "political" problems in securing agreements with government-owned telecommunications monopolies to provide such services.

Wright said that MCI is close to an agreement with France to provide telephone service from the U.S. "They the French decided they really had to interconnect with us," he said. MCI officials said the French government had realized that it would lose money if it allowed MCI to channel telephone traffic through Britain, where MCI already has an agreement to furnish service from the United States.

Revenue from the electronic transmission service will be shared with the Belgian government. MCI officials said that if the service reaches its goal of 80,000 to 100,000 transactions a year, it could pay for all start-up costs within nine or 10 months

MCI is the second-largest American long-distance carrier after AT&T. MCI has about 5 percent of the U.S. long-distance market, while AT&T has about 90 percent. MCI officials said the company serves about 2 million customers worldwide.