ISLAMABAD — Pakistani police have filed terrorism charges against the former prime minister, Imran Khan, authorities said Monday, escalating political tensions in the country as the ousted premier holds mass rallies seeking to return to office.

The charges followed a speech Mr. Khan gave at Islamabad on Saturday in which he vowed to sue police officers and a female judge and alleged that a close aide had been tortured after his arrest.

Mr. Khan himself has not publicly spoken about the latest charges against him. However, a court in Islamabad issued a so-called “protective bail” for Mr. Khan for the next three days, preventing police from arresting him over the charges, said a senior leader in his Tehreek-e-Insaf opposition party, Shah Mahmood Qureshi.

Hundreds of Tehreek-e-Insaf members stood outside Mr. Khan’s home on Monday in a show of support as the former premier held meetings inside. The party has warned that it will hold nationwide rallies if Khan is arrested while working to try to squash the charges in court.

Under Pakistan’s legal system, police file what is known as a first information report about charges against an accused person to a magistrate judge, who allows the investigation to move forward. Typically, police then arrest and question the accused.

The report against Mr. Khan includes testimony from a magistrate judge, Ali Javed, who described being at the Islamabad rally on Saturday and hearing Mr. Khan criticize the inspector-general of Pakistan’s police and another judge. Mr. Khan went on to reportedly say: “You also get ready for it, we will also take action against you. All of you must be ashamed.”

Mr. Khan could face several years in prison from the new charges, which accuse him of threatening police officers and the judge under the country’s sedition act, which stems from British colonial-era law. However, he’s not been detained on other lesser charges levied against him in his recent campaigning against the government.

The Pakistani judiciary also has a history of politicization and taking sides in power struggles between the military, the civilian government and opposition politicians, according to the Washington-based advocacy group Freedom House. The current prime minister, Shahbaz Sharif, likely will discuss the charges against Mr. Khan at a Cabinet meeting scheduled for Tuesday.

Mr. Khan came to power in 2018, promising to break the pattern of family rule in Pakistan. His opponents contend he was elected with help from the powerful military, which has ruled the country for half of its 75-year history.

In seeking Mr. Khan’s ouster earlier this year, the opposition had accused him of economic mismanagement as inflation soars and the Pakistani rupee plummets in value. The parliament’s no-confidence vote in April that ousted Mr. Khan capped months of political turmoil and a constitutional crisis that required the Supreme Court to step in. Meanwhile, it appeared the military similarly had cooled to Mr. Khan.

Mr. Khan alleged without providing evidence that the Pakistani military took part in an American plot to oust him. Washington, the Pakistani military and Mr. Sharif’s government have all denied the allegation. Meanwhile, Mr. Khan has been carrying out a series of mass rallies trying to pressure the government.

In his latest speech Sunday night at a rally in the city of Rawalpindi outside of Islamabad, Mr. Khan said so-called “neutrals” were behind the recent crackdown against his party. He has in the past used the phrase “neutrals” for the military.

“A plan has been made to place our party against the wall. I assure you, that the Sri Lankan situation is going to happen here,” Mr. Khan threatened, referencing the recent economic protests that toppled that island nation’s government.

“Now we are following law and constitution. But when a political party strays from that path, the situation inside Pakistan, who will stop the public? There are 220 million people.”