China, who won four of the six diving golds at the last championships in Perth, Australia, in 1991, look set for more medals as Lan compiled 355.35 points from his seven dives to qualify directly for today's semi-finals.
Lan is less than one point ahead of Germany's Boris Lietzow and Zimbabwe's Evan Stewart. Another German, Holger Schlepps, was fourth.
The top four go straight into the two-group semi- finals, while competitors who finish from fifth to 12th place in the preliminary round compete in quarter-finals for the last four semi-final places.
In the women's 10m platform event, Fu Mingxia, the Olympic champion, opened the defence of her title in commanding form.
Fu, who won the world title as a 12-year-old in 1991, dominated the preliminary round and outscored her nearest rivals by more than 30 points.
Mustapha Larfaoui, the president of the international federation, Fina, said that around 1,600 competitors from 102 countries were taking part, beating the previous record of 67 nations in Perth.
Fina has provided financial assistance to enable more countries with limited resources for swimming to come to Rome. The main swimming events begin next Monday.Reuse content