At least 21 states activating National Guard troops in capitals to prepare for possible attacks

At least 21 states are activating National Guard troops in preparation of possible attacks this week.

The FBI is warning that armed protests are being planned at all 50 state capitols as well on Capitol Hill after a group called for "storming" state, local and federal government courthouses and administrative buildings on Inauguration Day.

In Michigan, where armed militia breached the state Capitol in May, Gov. Gretchen Whitmer announced that the National Guard would be activated from Monday to Wednesday to provide additional security and crowd control measures.

"Last week's horrific scene on Capitol Hill was an attack on our democracy and shows that we need to be adequately prepared for acts of violence as President-elect Joe Biden is sworn in as our 46th President of the United States," said Whitmer, who faced a kidnapping plot against her in October.

Small protests took place in Ohio on Sunday, where Gov. Mike DeWine authorized 1,000 soldiers and airmen to provide assistance to federal authorities in Washington, D.C. Several hundred more are remaining in Columbus to protect citizens, public buildings and private property from "civil unrest," according to a statement from DeWine's office.

Other states where National Guard troops have been activated include California, Connecticut, Delaware, Florida, Georgia, Illinois, Kentucky, Maine, Minnesota, North Carolina, New Mexico, Nevada, Oklahoma, Oregon, Pennsylvania, Texas, Utah, Washington and Wisconsin.

Biden will be inaugurated as the 46th president of the United States on Wednesday in Washington, D.C.

The House of Representatives voted Wednesday to impeach Trump for incitement of insurrection on the Capitol, making him the first president to ever be impeached twice. The vote triggers a trial in the Senate, but senators will not return early for an impeachment trial prior to the inauguration.

ABC News' Alexandra Faul and Will McDuffie contributed to this report.