Pakistan's President, Mr Mohammed Rafiq Tarar, declared a state of emergency in Pakistan early this morning and simultaneously announced "temporary" restrictions on foreign exchange transactions, officials said.
The president declared the state of emergency under constitutional guidelines which call for a state of emergency to be declared when Pakistan's security falls under "threat of external aggression", the official Associated Press of Pakistan news agency said.
Under a separate decree President Tarar imposed "temporary restrictions" on selling, withdrawing or transferring money from foreign currency accounts, on which there had been expected to be a run. Pakistan has declared today as a holiday for local and foreign commercial banks.
Earlier Pakistan yesterday explained its decision to detonate five nuclear test bombs, saying it had now "evened the account with India".
The Prime Minister, Mr Nawaz Sharif, said the tests, at a site in the south-western province of Baluchistan near the border with Afghanistan, were successful.
One Pakistani official said more tests were likely in coming days.
Tens of thousands of people poured on to the streets of Pakistani cities to celebrate the event. The blasts followed five nuclear explosions by neighbouring India two weeks ago.
"We have evened the account with India," Mr Sharif declared, adding that it was an "auspicious day of historic importance" for Pakistan, which became the first Muslim country to declare itself a nuclear power.
He said India had ignored Pakistan's numerous non-proliferation proposals and expressed new disappointment at the international reaction to India's tests.
"We were not in a hurry to respond, but I do not represent a cowardly nation," he said. "We have rejected all offers and incentives to dissuade Pakistan from conducting its nuclear tests."
Pakistan will start an austerity drive to counter likely sanctions, Mr Sharif said.
He said he would leave his official prime minister's residence so it could be used for social welfare purposes and would give up meals if necessary.
He vowed the nation would surmount the economic challenge of any sanctions, which he said would be unjust.
Pakistan took China into its confidence before conducting the nuclear tests, the country's official news agency APP reported. It said Pakistan also consulted Islamic allies Iran and Saudi Arabia.
The Australian Geological Survey Organisation said it had registered Pakistan's tests as one explosion with a magnitude of 5 on the Richter scale. It said the explosion had a yield of between five and 20 kilotonnes.
The US atomic bomb dropped on Hiroshima, Japan, in 1945 was around 20 kilotonnes.
Pakistan's nuclear tests drew immediate protests from survivors of the US atomic bombs dropped on Hiroshima and Nagasaki at the end of the second World War in 1945.
"What we have feared the most after India's underground tests has happened," said Mr Koshiro Kondo, secretary-general of the liaison council of organisations of atomic-bomb survivors in Hiroshima.
"We kept on saying that Hiroshima is not a problem of the past. I regret that it has not been correctly understood," he said. "As hibakusha [atomic-bomb survivors], we will patiently appeal to the world for the abolition of nuclear weapons."
Mr Hirotami Yamada, who heads a similar council in Nagasaki, said: "What we have feared has turned into a reality.
"It is necessary to keep on sending our message of fierce criticism against possession of nuclear weapons so that the nuclear tests this time would be prevented from helping a revival of the nuclear arms race."
More than 210,000 people perished in the two Japanese cities following the first and only nuclear attack in history.