WASHINGTON, D.C. (NBC) - A large rally to commemorate 9/11 is roaring through the Washington, D.C., area.

Thousands of bikers with the group "Two Million Bikers to D.C." are snarling traffic on the Beltway with a ride honoring the victims of the Sept. 11, 2001, terrorist attacks, and the members of the military called to serve after the attacks.

On their Facebook page, in all caps, the group said they will stand by the Constitution and Bill of Rights, adding that they're "against any fundamental transformation of America."

Organizers tried to get a last-minute permit to close some intersections near monuments and on Capitol Hill in D.C., but the National Park Service denied that request. According to US News and World Report, park service spokesperson Carol Johnson said allowing the road closures "would cause a severe service disruption of traffic."

However, permits are not required to hold a rally in the city. It will be up to individual riders whether they choose to enter the city, WTOP reported.

On their Facebook page, the group apologized for the gridlock they'll likely cause.

"What could have been a one or two hour ride through will now likely be an all day event," the organizers said in the Sept. 6 post.

The ride will begin at Harley Davidson of Washington in Fort Washington, Md., at 11 a.m. The group did not release a route, citing "security purposes."

One group of participants is expected to make their way up I-95 North in Stafford at around 9 a.m. They should reach D.C. sometime after 11 a.m. I-95 North at exit 143 in Stafford County was closed for about 15 minutes as motorcyclists began their trip to D.C.

Shortly after the rally got underway, an accident involving a motorcycle was reported on the Beltway between Connecticut Avenue and Rockville Pike. There's no word on the condition of the motorcyclist or the circumstances surrounding the accident.