His promise came in a last meeting Thursday morning with the cardinals who will pick his successor, almost certainly from within their own ranks.
"I will continue to serve you in prayer, in particular in the coming days, so that you may be touched by the Holy Spirit in the election of a new pope," he said.
His words appeared designed to answer concerns that the presence of a former pontiff might lead to confusion or competing loyalties once the new pope is installed.
Benedict told the cardinals it was a "joy to walk with you" during his nearly eight tumultuous years at the head of all Catholics worldwide.
Another Vatican spokesman, the Rev. Thomas Roscia, said he believed 144 cardinals had attended Benedict XVI's farewell to them as pope. That includes both cardinal-electors, who are under the age of 80, and cardinals who are not eligible to vote for the next pope.
Not all the 115 cardinals eligible to vote were present, Lombardi said.
Two cardinals are suffering ill health, making their attendance uncertain, although arrangements may be made to enable them to vote, Roscia said.
The Vatican has said it wants to have the next pontiff in place in time for the week of services leading up to Easter Sunday on March 31.
A series of meetings to set the timetable for the conclave -- the closed-door assembly to elect a new pope -- will begin early next week, said Lombardi. The cardinals will receive the formal invitation to attend on Friday.
The meetings, known as general congregations, bring together all the cardinals, electors and non-electors, before the conclave begins. They are intended to be an opportunity to reflect on the current state of the church.
In their meeting Thursday morning, the cardinals gave Benedict a standing ovation, and then one by one each met Benedict to say a final few words.
Cardinal Daniel DiNardo of Texas said at a news conference held with fellow U.S. cardinals that it had been "a very moving moment with Pope Benedict."
"There was a note of sadness in saying farewell to this man who has been our spiritual father for the last eight years," said Cardinal Sean O'Malley of Boston.
"At the same time it was very edifying to see how much people love him and respect him."
Cardinal Roger Mahony, the retired archbishop of Los Angeles, tweeted after meeting Benedict that he had asked Benedict to pray for the people of Los Angeles. "He grasped my hand and said 'Yes'!!" Mahony said.
The current Catholic archbishop in Los Angeles earlier this month disciplined Mahony for his mishandling of "painful and brutal" allegations of sexual abuse by priests. Mahony's decision to travel to Rome to take part in the election of the new pope has been controversial because of that.
DiNardo and O'Malley said they would pray for guidance in choosing the new pope.
"I consider it one of the most important activities that I will be engaged in as a priest and a cardinal," said O'Malley, for whom the conclave will be his first.
"I think the discussions that we will have in the congregations will be the most important intellectual preparation that we have -- certainly the spiritual preparation has already begun.
"Our people back home and throughout the world are all praying that we will be guided to be able to choose the very best person to lead us."
Cardinals are forbidden to communicate with the outside world -- now including by Twitter -- during the conclave, held within the Sistine Chapel. There is no Internet access inside Santa Marta, where the cardinals will stay during the conclave, Lombardi said.
Cardinal Gianfranco Ravasi of Italy, tipped as a possible future pope, tweeted Thursday morning that he would be away for a few days.
A number of other cardinals, including Ghanaian Peter Turkson, also considered a frontrunner, and New York's Timothy Dolan are also present on Twitter.