The western Canadian province of Alberta tonight announced it was planning a gradual cutback in shipments of oil to eastern Canada, a direct challenge to the national energy policies of Prime Minister Pierre Trudeau's government.

The move by Alberta brings Canada a step closer to a widely feared showdown over resource jurisdiction between the Trudeau administration and Alberta, which produces most of Canada's oil. t

Alberta's decision, announced by provincial Premier Peter Lougheed in an televised address, came two days after the Trudeau government revealed a sweeping energy program including a tax on natural gas production that Lougheed said is unacceptable.

Lougheed said his province, which sends 1 million barrels a day of crude oil to eastern Canada, plans to stage cutbacks of oil production over the next nine months, reducing output 15 percent by the end of the period. When fully implemented, this would reduce shipments to eastern Canada by a total of 180,000 barrels a day, requiring the country to double its current imports of high-priced crude oil.

But, in a significant concession, Lougheed avoided threatening Canada's populous, energy-consuming eastern region with oil shortages as a result of Alberta's actions.

"If there becomes any shortage problem in Canada, we will suspend" the production cutback, he said, suggesting that a shortfall can also be met by increased imports.

The threat of severe supply cutbacks, implying hardships for consumers and industry in Ontario, Quebec and other eastern Canadian provinces, has been Lougheed's strongest potential weapon in a continuing battle with Trudeau over control of resources and related constitutional issues.

Alberta and other western provinces have been fighting to establish local control of their oil and gas resources. Their aim is to build a strong industrial base before their nonrenewable resources run down later in this decade.

While the provinces have ownership of their resources under Canadian law, the central government has invoked its powers of taxation to increase its share of revenues from energy sales in order to finance a broad program designed to make Canada self-sufficient in energy by 1990.

Lougheed also announced that the province would take up a public relations campaign against Ottawa's energy program, a legal challenge against the new natural gas tax and moves to postpone the development of new synthetic oil projects.

Lougheed said the "measured response" announced tonight was calculated to avoid forcing Ottawa to invoke its orriding constitutional powers under which the national government could declare an emergency and take control of the production of energy resources.