Quote:“I am very excited that there’s no oil in the Gulf of Mexico,” Kent Wells, a senior vice president for BP, said about the flow during a teleconference on Thursday, “but we just started the test and I don’t want to create a false sense of excitement.”

Oil stopped flowing at 2:25 p.m. local time, Mr. Wells said, when engineers closed the choke line, the final seal of the well. Engineers and scientists will now examine the results of the tests every six hours to determine the pressure levels.

The view one mile beneath the gulf on BP’s continuous live video feed was conspicuously calm, devoid of the clouds of crude oil that had been billowing since the disaster first occurred in April. Despite the long-anticipated moment, officials involved in the spill effort, including President Obama, were quick to downplay the development as a temporary measure.

“I think it is a positive sign, we’re still in the testing phase and I’ll have more to say about it tomorrow,” President Obama said in response to a shouted question at the conclusion of a news conference devoted entirely to the passage of the financial regulatory bill.

"We’re encouraged by this development, but this isn’t over," Thad W. Allen, the retired Coast Guard admiral who is overseeing the federal response to the spill, said in a statement on Thursday. "It remains likely that we will return to the containment process using this new stacking cap connected to the risers to attempt to collect up to 80,000 barrels of oil per day until the relief well is completed."

Earlier on Thursday, the national incident commander, Thad W. Allen, said that closing the well off using the containment cap would only be an interim measure, and that the company must still complete the relief wells it is working on in order to seal the well for good.

The test commenced after two days of delays while BP fixed a leak in the equipment that engineers discovered on Wednesday night. Engineers replaced equipment on the tight-sealing cap that has been placed at the top of well, 5,000 feet under water, said Kent Wells, a senior vice president of the company. The equipment, part of a choke line that was the last valve to be closed before the pressure test could begin.

BP said that its three-ram capping stack was closed, “effectively shutting in the well and all sub-sea containment systems.”

Live feeds of video images from the undersea well clearly showed that the release of oil had had been completely halted.

Mr. Allen, clarified the role of the cap in his news conference on Thursday morning, saying that this mechanism was never meant to be the ultimate solution to closing the well.

Mr. Allen called it a “precursor” to containment, making it possible for the gushing crude to be captured through four different systems that together can keep up with the estimated rate of flow, which the government now puts at 35,000 to 60,000 barrels a day. If all goes well, it may also be used to seal the well completely for brief periods.

“I don’t want to reverse the priorities here, because the priority was to contain and stop the flow of oil,” he said, “but the design of the cap itself, if we can withstand the pressures and the well bore stays intact, presents the opportunity to shut the well in, which will give us the ability to abandon the site in a hurricane, so it’s a two-for if we can do it.”

The test involves closing all the valves on the new cap, which was installed earlier in the week, to increase pressure in the well so that BP can assess its condition over the length of the well bore, which extends 13,000 feet below the seabed.

Mr. Allen likened the process to putting a thumb over the end of a running garden hose. If the pressure does not rise as a result, that means there is a leak somewhere. In the case of the well, if the resulting pressure is high, that means the well bore is intact, he said.

“We have been slowly using mechanisms to close off the hose,” Mr. Allen said.

With those mechanisms all but closed off by Thursday morning, BP prepared to start watching the pressure readings. If all goes well and the pressure remains high, the test will continue for 48 hours. But even then, the oil will not be completely stopped, Mr. Allen said, as BP evaluates the test results with seismic readings beneath the sea.

The testing procedure has been plagued with delays. On Tuesday, the government asked BP to postpone the test for 24 hours while scientists reviewed the procedures. Officials were concerned about the possibility that the test itself might damage the well. The decision was made to allow the test to go ahead on Wednesday, with some modifications; the discovery and repair of the valve leak set it back to Thursday.

At the White House on Wednesday, Robert Gibbs, the press secretary, said that Energy Secretary Stephen Chu and others had been involved in the review, asking BP about the possible effect of the test on the well’s condition.

Mr. Gibbs described the review as “a series of steps” that were being taken “in order to ensure that what we’re doing is being done out of an abundance of caution to do no harm.”

Among the concerns was that if the well was damaged during the test, oil and gas might leak from the seafloor around the well rather than up through the well bore as it is now.

While the test is being conducted, the drilling of relief wells — considered the ultimate solution to stopping the gusher at its source — will be halted as a precaution, BP said.

Viewed from a Coast Guard cutter about a mile away, the well site was a floating city on Wednesday, with scores of vessels scattered nearby in calm conditions. Activity was centered on the spot where the Deepwater Horizon drilling rig exploded and burned in mid-April, sinking two days later and leaving the well it was working on broken open.

Another huge drill rig, the Development Driller III, was visible about a mile away from the site on Wednesday, working on the first relief well. A nearly identical rig, the Development Driller II, was working on a backup relief well that is not as far along. Supply boats attended both rigs, with long sections of casing pipe lying ready on their decks.

At about 3:30 p.m. Wednesday, the tongues of fire from the collection ships that had marked the site for weeks went out, only to be restarted later when the valve leak was discovered.