The first of the 1,900 guests arrived at 3:15 a.m. ET, but there were already thousands of eager royal wedding watchers lining the streets three hours before the ceremony was to begin.
People have come from far and wide, one lady even painted her face blue, saying she had come as the engagement ring.
At Hyde Park corner, a crowd of 180,000 is expected to gather around the large screens to watch Kate Middleton and Prince William tie the knot.
Thousands of people camped out overnight on the procession route; and there is also the specially constructed Camp Royale in Clapham Common.
More than 1,500 soldiers, sailors and air crew will be on duty to line the couple's procession route between Westminster Abbey and Buckingham Palace, just under a mile away.
Also, an additional 5,000 uniformed and undercover police have been called in to be on alert for threats from Irish dissidents, Muslim extremists, anti-monarchists, royal obsessives and anyone who might be drunken or disorderly.