It is believed that a settlement on the site of modern-day Paris was founded about 250 BC by a Celtic tribe called the Parisii, who established a fishing village near the river Seine. The Île de la Cité was traditionally assumed to be the location of the settlement, but this theory has been recently brought into question. Recent archeological finds indicate that the Paris region's largest pre-Roman settlement may have been in the present-day suburb of Nanterre.
Paris's lands were prosperous, and occupied a strategic position for controlling river shipping and commerce. The area came under Roman control after the revolt of 52 BC when Vercingetorix led a Celtic uprising against the Romans under Caesar. The town sided with the rebels and was said to have contributed 8,000 men to Vercingetorix's army. It was garrisoned by Vercingetorix's lieutenant Camulogenus, whose army camped on the Mons Lutetius (where the Panthéon is now situated). The Romans crushed the rebels at nearby Melun and took control of the entire region. By the end of the same century, the Île de la Cité and Left Bank Sainte Geneviève Hill became the centre of a new Roman settlement called Lutetia.
Under Roman rule, the town was thoroughly Romanised and grew considerably. It was, however, not the capital of its province, Lugdunensis Senona—that role was played by Agedincum (modern Sens, Yonne). It was Christianised in the 3rd century when St Denis became the city's first bishop. The process was not entirely peaceful—in about 250 St Denis and two companions were arrested and decapitated on the hill of Mons Mercurius, thereafter known as Mons Martis (Martyrs' Hill, now Montmartre).
Lutetia was renamed Paris in 212, after the local tribe, but the rest of the 3rd and 4th century was wracked by war and civil unrest. The city came under attack from barbarian invaders, prompting the construction of a defensive city wall. In 357 the Emperor Constantine's nephew Julian arrived in Paris to become the city's new governor. Although his uncle had declared Christianity the official religion of the Empire, Julian "the Apostate" strove to roll back its advance. He became emperor in 361 but died in battle only two years later.
Roman rule in northern Gaul effectively collapsed in the 5th century. In 451, the region was invaded by Attila the Hun, prompting fears that Paris would be attacked. According to legend, the city was saved by the piety of Sainte Geneviève and her followers, whose prayers for relief were answered when Attila's march turned away from Paris to the south. St. Geneviève remains Paris's patron saint to this day.