The president-elect and his wife took a police convoy to the church at 8.40am US time.
The inauguration day traditionally starts with a morning prayer at St Johns Episcopal Church across from the White House in Washington.
It all begins today! The movement continues- the work begins
Mr Trump said on Twitter: "It all begins today! I will see you at 11:00 A.M. for the swearing-in. THE MOVEMENT CONTINUES – THE WORK BEGINS!"
He and vice president elect , Mike Pence, will take the oath of office at 5pm GMT outside the domed US Capitol, with US Chief Justice John Roberts presiding.
The 70-year-old stayed the night at an official guest house near the White House and then attended a morning prayer service at St. John's Episcopal Church, a block away.
The Obamas welcomed the Trumps to the White House
Barack Obama welcomes Melania Trump to the White House
Mr Trump, wearing a dark suit and red tie, was accompanied by his wife, Melania, clad in a classic-styled, powder blue ensemble, and his family.
Streets near the president's home were blocked to traffic by empty buses and dump trucks or temporary pedestrian security checkpoints where law enforcement officers and National Guard troops checked people's bags.
Checkpoints around the National Mall in front of the Capitol opened early to begin admitting guests, some of them wearing red caps bearing Mr Trump's "Make America Great Again" slogan.
Melania and Trump leave the church
Today Trump will become the 45th president of the US
Trump is at a church close to the White House in Washington
People gather to watch the inauguration in Washington DC
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They were barred from bringing selfie sticks, coolers for beverages, and long umbrellas despite the rainy weather.
Most of the area was orderly, but about 100 protesters shouted slogans near one checkpoint and linked arms to block people from entering.
Police in riot gear pushed them back into an intersection to allow people attending the inauguration to reach the checkpoints.
More to follow…