On the eve of the country's first national elections in eight years, President Mohammed Zia ul-Haq warned political opponents yesterday that his troops will be on standby to ensure that the voting runs smoothly. He also said martial law would continue past the elections.

In a nationally televised address Zia said, "I expect the administration and law enforcing agencies to maintain law and order at all costs," The Associated Press reported. The warning appeared aimed at the underground, 11-party Movement for the Restoration of Democracy. All but one of its leaders have been detained for urging an election boycott.

At a news conference later in Rawalpindi, Zia said he will begin lifting martial law after he hands over power to an elected government -- but not until he is satisfied that there is political stability.

"Martial law will be lifted in stages after we have amended the constitution fully," he said. "It is not a switch-on, switch-off sort of thing."

A Pakistani journalist who reports for several international news organizations was released 12 hours after he was arrested. Iqbal Jaffery, who contributes to United Press International and others, had reported the arrests of hundreds of opposition politicians in the campaign.

Jaffery, 42, said he was released after meeting with Karachi commissioner Syed Sardar Ahmad, the city's top civilian administrator, who apologized.

In London, The Observer quoted Zia as saying that Pakistan has succeeded in producing small amounts of enriched uranium but that the project aims to produce nuclear power, not bombs.