Until 1992, the Mughal mosque, built by the Mughals in the 16th century, stood on the site of the temple.
That year, a temple was demolished by a group of Hindus and protests erupted, killing nearly 2,000 people across the country.
Hindus believe that the god Ram was born at that spot and that there was a Ram temple there before the Mughal mosque was built.
Since the demolition of the mosque, Hindu nationalist politicians have promised to build a temple to Ram there.
The movement to build a temple here helped Modi's Bharatiya Janata Party, now in power, to enter the political arena with strength in the 1990s. The three-story temple cost $217 million to build.
A 130 cm. tall statue of Ram was also made for the temple.
In India, where general elections will be held in a few months, most of the opposition will not attend the inauguration ceremony, arguing that Modi is using the temple for his political interests.
At today's "manifestation of life energy" ceremony, religious words will be repeated and religious rituals will be performed around the fire.
Through these, it is believed, sacred life will be breathed into statues or photographs of the god Ram. Modi will only open the first floor of the temple today.
Some Hindus, however, consider it sacrilegious to hold ceremonies in an unfinished temple. Officials expect the temple to attract 150,000 visitors a day once it is completed.
So the city is preparing for this with major infrastructure investments. A new airport and train station have opened in recent weeks, and several new hotels have been built.
The government has allocated $3.85 billion to transform the tranquil city on the Saryu River, a tributary of the Ganges.
However, some residents told the BBC that not only their homes and businesses but also some religious buildings have been demolished as part of the massive transformation.
