Excessive force by the police during an arrest violates the Fourth Amendment to the U.S. Constitution. A suspect who has been a victim of excessive force may have a viable lawsuit against the arresting officers and even the municipality that employs them.
A "tort" is a legal term for a civil wrong (as opposed to a criminal wrong) that resulted in some kind of injury to the plaintiff. Many civil claims against police officers involve the torts of assault and battery. Occasionally, these claims involve the tort of negligence. Generally, these claims will be evaluated under a state's tort claims act.
Most lawsuits against police officers involve the Civil Rights Act of 1871. (42 U.S.C. § 1983.) A Section 1983 claim alleges that the defendant, "under color of law," violated the plaintiff's constitutional rights. The use of excessive force constitutes a valid claim under section 1983 because it violates the Fourth Amendment prohibition against "unreasonable seizures." (For information about other kinds of police lawsuits, see Police Conduct and Emotional Distress and Taser Lawsuits Against the Police.)
Police officers are generally allowed to use whatever force is necessary to make an arrest or defend themselves. In most jurisdictions, when a jury has to decide whether an officer used more force than was necessary to make an arrest, the judge instructs it to consider what a reasonable person with the officer's knowledge would have deemed necessary under the circumstances.
So, an arresting officer is allowed to use more force to arrest a resisting suspect than if the suspect were compliant, and may use deadly force if threatened with death or great bodily harm. The amount of force an officer may lawfully use against a fleeing suspect depends on whether the person appears to have committed either a felony or a misdemeanor. (For more information on resisting arrest, see Resisting Arrest: Laws, Penalties, and Defense and Resisting Arrest When Police Use Excessive Force.)
Whether an officer's use of force was excessive is so dependent on the facts that appellate courts often defer to juries' conclusions in that regard.
In a civil suit, the burden is usually on the plaintiff to prove liability by a "preponderance of the evidence" (meaning more likely than not). A defendant—in this case, an officer—who raises a defense of justification must prove by the same standard that there was a legal excuse for the conduct in question. (The preponderance-of-evidence standard is much lower than that in a criminal case: "beyond a reasonable doubt.")
But many states treat excessive force cases somewhat differently than typical lawsuits. In some jurisdictions, there is a presumption that the officer acted with the necessary level of force that the plaintiff must overcome. Additionally, some impose a higher burden of proof than "preponderance of the evidence," instead, requiring the plaintiff to prove a claim of excessive force by "clear and convincing evidence" (a standard higher than "by a preponderance of evidence" but lower than "beyond a reasonable doubt").
All states agree that the plaintiff being guilty of the crime for which the officer arrested him isn't a valid defense for the officer. But, by the same token, a plaintiff who can prove innocence is more likely to be able to show that the officer's use of force wasn't necessary.
A victim who sues a law enforcement officer for a civil tort will likely face an obstacle known as immunity. Many states have statutes that provide immunity to public employees who are performing discretionary duties (like making an arrest). But, depending on the state, this protection might not apply when officers act in bad faith or outside the scope of their duties (such as excessive use of force).
Another potential challenge for citizens who want to sue the police department and the city or county is governmental immunity. States are immune from suit by private citizens in federal court under the 11th Amendment to the U.S. Constitution. Additionally, many states have laws that protect municipalities from liability in equivalent suits. Ultimately, whether the government has a viable "immunity" claim depends on the facts of the case and the jurisdiction it's in.
In a Section 1983 claim, police will typically raise the defense of qualified immunity, which protects an officer from liability for civil rights violations as long as the officer's conduct did not violate a clearly established right.
The law governing excessive force suits can vary greatly from state to state and from state to federal court, especially on issues like the burden of proof and immunity. If you or someone close to you has been the victim of excessive force by the police, seek the help of an experienced civil rights attorney. A knowledgeable lawyer will be able to advise you of the applicable law and protect your rights.