Iraqi forces seized a large cache of explosives and arrested suspected insurgents allegedly planning to target government ministries Tuesday, in a crackdown across the capital that brought parts of the city to a standstill.

BAGHDAD — When Baghdadis awoke Tuesday to find their streets sealed off and the Iraqi capital under virtual lockdown, the rumors began to fly. Army officers had staged a coup in the Green Zone, one version said. No, members of the Baath party loyal to the former regime had taken over, according to another.

At midday, officials appeared on television to try to calm the city. “The security forces can’t stage a coup. Our security forces are professional,” military spokesman Mohammed Askari told a news conference. “The era of coups is gone.”

Rather, he said, the government ordered the lockdown to foil a plot involving car bombings and suicide attacks against civilian and government targets.

In a later statement, security forces said they had detained 25 people and confiscated 440 pounds of TNT, 440 pounds of C4 explosives, more than 65 gallons of ammonium nitrate and 60 mortar rounds.

There may well have been such a plot: Parliamentarians were told by the authorities that the Iraqis had received a tipoff from the U.S. military that around six booby-trapped cars had arrived in Baghdad from the Syrian border area, and were to be used to target sites such as government ministries, Kurdish lawmaker Mahmoud Othman said.

U.S. military spokesman Maj. Gen. Stephen Lanza would not comment directly on the U.S. role, but said U.S. forces “have a very good relationship with the Iraqis, in both sharing information and in our efforts to support the conduct of operations” with Iraqi security forces.

Such attacks have been widely predicted in the run-up to March elections and in the wake of three recent similar attacks that left hundreds of people dead. Whether the alleged plot had been fully thwarted was open to question, however.

The amounts of explosives uncovered barely added up to one of the bombs used in the earlier attacks, each of which contained around 2,000 pounds of explosives. The government did not specify whether the security forces had found the bombs purported to be circulating.

But the panic showed just how jittery the city is as the election approaches. Though most roads were reopened by midmorning, schools were closed and some neighborhoods were sealed off into the evening. By nightfall, streets that would normally be bustling with traffic were almost deserted.

“People are feeling very nervous about the security situation and also about the political situation, which is getting more complicated every day,” said Nabil Salim, a political scientist at Baghdad University.

There also is a high level of paranoia about the threat posed by Baathists, who ruled under Saddam Hussein. The government has helped fuel the fears by repeatedly warning that the outlawed group is plotting to return to power and blaming it for the bombings.

One of the most widely circulated rumors Tuesday was that a leading Sunni politician, Saleh Mutlak, had been assassinated inside Baghdad’s Green Zone. Days earlier, a committee charged with checking that candidates don’t have ties with Baathists had recommended that Mutlak be barred from taking part in the elections.

Mutlak showed up alive and well at the Iraqi parliament, after being awakened early by people calling to see if he was OK. He laughed off the rumors. “They tried to assassinate me politically, and now physically,” he said.