A California man who stormed the US Capitol, opened the doors to other rioters and sat in the Senate chair of Mike Pence pleaded guilty to a federal charge on Thursday.

Christian Secor, 23, of Costa Mesa, entered the plea in a Washington court to obstructing an official proceeding.

More than 100 police officers were injured on 6 January 2021, when a mob of supporters of Donald Trump attacked the Capitol while Congress was holding a joint session to certify now-President Joe Biden’s 2020 electoral victory.

Secor was a University of California, Los Angeles student at the time who had founded a far-right conservative student group called America First Bruins, authorities said.

According to court documents, Secor sent a text message on the day of the 2020 election stating: “We’re gonna win bigly and if we don’t we’re taking this ship down in flames,” the US Department of Justice said in a statement.

He sent another message on 5 January 2021, telling an acquaintance that he had brought a gas mask to Washington and “wouldn’t be surprised if conservatives just storm the police and clobber antifa and the police but that’s wishful thinking”.

In his plea agreement, Secor acknowledged that the next day, he joined a mob that poured on to Capitol grounds, climbed scaffolding to reach an upper terrace, entered and walked through the building, including the offices of the speaker, Nancy Pelosi, helped other rioters push open doors barred by three police officers so others could enter, and ended up by sitting in the vice-president’s Senate chamber seat before leaving.

He later tweeted that “one day accomplished more for conservatism than the last 30 years”.

He was arrested on 16 February.

In return for his plea, federal prosecutors agreed to drop other charges, including assaulting a police officer.

Secor technically could face up to 20 years in prison and a fine of up to $250,000 when he is sentenced in October.

However, sentencing guidelines call for 21 to 27 months in prison, or 53 to 61 months in prison if Secor is found to have caused injuries or property damage, according to the plea agreement.

More than 790 people have been charged with federal crimes related to the Capitol riot. Nearly 300 of them have pleaded guilty, mostly to misdemeanors. Over 170 of them have been sentenced.

More than a dozen defendants have pleaded guilty to felonies and they have been sentenced to prison terms ranging from six months to five years and three months.