The operation, a pilot project, will initially use 21 coaches, but Colin Child, finance director, said National Express could have 400 vehicles on Polish roads within three years if the service is success.
Poland, described by Mr Child as being at the crossroads of Europe, may provide the launch pad for services throughout Eastern Europe. Poland's growing economy and openness to Western businesses made it a suitable place to begin services, he said.
National Express has invested pounds 2.4m in new coaches, and will use local operating companies for servicing and maintenance.
The company has been growing its European routes, but this is the first time National Express has started a domestically-based service outside Britain.
Poland has few long-distance travel services and moving from one city to another usually means a series of short journeys by bus or train.
Services across Europe are proving a big growth market for coach companies, especially within the European Union. Travel has become easier with the removal of trade barriers, and anti-competitive legislation has enabled firms to compete more effectively with domestic carriers. Last year National Express bought Eurolines Nederland, a partner of its Eurolines UK subsidiary.