Who can arrest the President?

All of this talk is in reference to U.S. Chief Executives.

I’m curious as to whether any other Head of State has been tried for a crime while he/she was still in office.

Charles I, of course.

Umm… doesn’t Double Jeopardy apply there? You can’t be prosecuted twice.

Short answer: Not according to the Office of Legal Counsel.

No, the Constitution says in so many words:

“Judgement in cases of impeachment shall not extend further than to removal from office, and disqualification to hold and enjoy any office of honor, trust, or profit under the United States: but the party convicted shall nevertheless be liable and subject to indictment, trial, judgement and punishment, according to law.”

Actually, the Impeachment Judgments Clause, on its face, deals only with convictions. Based on its mention of convictions and neglect of acquittals, some have argued that the doctrine of expressio unius requires a double jeopardy-like bar on prosecutions of those acquitted by the Senate. http://www.usdoj.gov/olc/expresident.htm#back_5

There is a detailed discussion of the issues in the section of the OLC memo I just linked to. While the OLC ultimately found arguments based on the Impeachment Judgments Clause and Double Jeopardy Clause unconvincing, it noted that it was a closer case one might think on first consideration:

http://www.usdoj.gov/olc/expresident.htm#back_119