Rescuers faced a complex task as the structure continued to give way as they tried to get near the trapped worker, who officials said was alive, through a first-floor window, forcing their retreat on telescopic aerial ladders in a cloud of smoke.
Another approach on two ladders was also aborted and a drone sent up in their stead.
As dusk approached, firefighters lifted on a crane used giant tubes to suck rubble out of the second-floor window.
They continued the work into the night.
The 29-metre-high Torre dei Conti stands near one of Rome's tourist hotspots.
It is located halfway along the Via dei Fori Imperiali, the broad avenue that leads from central Piazza Venezia to the Colosseum.
Rome's top law enforcement official, Prefect Lamberto Giannini, said there were "signs of life" from the trapped worker, and that firefighters had managed to give him some protective cover during an earlier rescue attempt.
Three workers were rescued unharmed after the initial midday collapse, firefighter spokesman Luca Cari said.
Another worker, age 64, was hospitalised in critical condition; RAI state TV reported he was conscious and had suffered a broken nose.
No firefighters were injured in the ongoing operation.
Giannini called the situation "very complex" and said that other equipment was being brought in.
"It will be a very long operation. We must try to save this person while mitigating the enormous risks to the people who are trying to save him," Giannini told reporters.
Hundreds of tourists had assembled to watch as firefighters used a mobile ladder to bring a stretcher to the upper level of the Torre dei Conti during the first rescue attempt.
Suddenly, another part of the structure partially collapsed, sending up a cloud of debris and forcing firefighters to quickly descend on the ladder.
The first collapse struck the central buttress of the structure's southern side and caused the partial collapse of an underlying sloped base.
The second collapse hit part of the stairwell and roof, cultural heritage officials said in a statement.
The tower, which has been closed since 2007, is undergoing a 6.9 million euro ($A12.1 million) restoration, that includes conservations work, the installation of electrical, lighting and water systems and a new museum installation dedicated to the most recent phases of the Roman Imperial Forum, officials said.
Before the latest phase was started in June, structural surveys and load tests were carried out "to verify the stability of the structure, which confirmed the safety conditions necessary" to proceed with work, including asbestos removal, officials said.
The current work, carried out at a cost of 400,000 euros, was just about complete.
Italian prosecutors arrived at the scene as the rescue operation was underway and were investigating for possible charges against negligent disaster and negligent injuries, Italian media reported.
It is common in Italy for investigations to begin while an event is ongoing and before possible suspects are identified.
The Torre dei Conti was erected by Pope Innocent III for his family in the early 13th century and was originally twice as high but was scaled down after damage from earthquakes in the 14th and 17th centuries.
Rome Mayor Roberto Gualtieri and Italian Culture Minister Alessandro Giuli were at the scene but did not speak to reporters.
"The firefighters risked their lives with the second collapse. We are worried for the worker who remains trapped," said Federico Mollicone, chairman of the parliamentary culture committee.
with AP
