Reuters - The chaos inside the U.S. Capitol this week was a result of a catastrophic failure to prepare, according to law enforcement officials that protect the legislative complex overrun by a mob.

The siege of the Capitol, home to both the U.S. Senate and the House of Representatives, represents one of the most serious security lapses in recent U.S. history, current and former law enforcement officials said.

Events such as a presidential inauguration involve detailed security plans by numerous security agencies but far less planning goes into protecting the joint session of Congress that convened earlier this week to ratify the results of the 2020 presidential election, the officials said.

That lapse came despite warning signs of potential violence by hardline supporters of U.S. President Donald Trump.

Security was initially handled almost entirely by the U.S. Capitol Police - a 2,000-member force under the control of Congress and dedicated to protecting the 126-acre Capitol Grounds.

Other agencies of the U.S. federal government's security apparatus didn't arrive in force for hours as rioters besieged Congress. The Capitol is a short walk from where Trump in a speech railed against the election just before the riot began, calling the vote an "egregious assault on our democracy" and urging his supporters to "walk down to the Capitol" in a "Save America March."

The counting of the electoral votes of the presidential election by Congress, normally a formality, was preceded by weeks of threats on social media that planned pro-Trump protests could descend into violence.

Capitol police didn't request advance help from other federal agencies to secure the building, according to one senior official. National Guard reinforcements, summoned by the city's mayor, were not mobilized until more than an hour after protesters had first breached the barricades.

These agencies were deployed by the Trump administration during last summer's police-brutality protests in Washington and elsewhere in the U.S.

Capitol police chief Steven Sund said Thursday his agency was conducting a "thorough review of this incident, security planning and policies and procedures." The agency, he said, had a "robust plan" to address "anticipated first-amendment activities."

"But make no mistake - these mass riots were not first-amendment activities. There was criminal riotous behavior," Sund said. "The actions of the Capitol police officers were heroic given the situation they faced."

Officers were attacked with lead pipes, chemical irritants and other weapons, the chief said. In a separate video news conference, Democratic House Rep. Tim Ryan said up to 60 police officers were injured and 15 remained in the hospital. One is in critical condition. Many had head injuries after being bludgeoned by rioters, Ryan said.

Late Thursday House Speaker Nancy Pelosi, a Democrat from California, called for police chief Sund's resignation. She said she had already received a notice from House sergeant-at-arms Paul Irving - who is responsible for security in the chamber - that he would resign.

"There was a failure of leadership at the top of the Capitol police," she said.

She added that other federal agencies also deserved a share of the blame for the failed planning and response to the Capitol siege. "It goes beyond the police," she said. "It goes to many other elements of the executive branch."

A senior federal law enforcement official familiar with the planning to protect other federal sites Wednesday, including the grounds where Trump spoke, said he was shocked that the Capitol police were not better prepared.

"It looked like the Keystone Cops out there," said the official, who spoke on condition of anonymity. "It should have never happened. We all knew in advance that these people were coming, and the first order of policing is presence."