Coach Warren Gatland accepted the second challenge while captain Sam Warburton accepted the third in front of Te Whare Runanga, the carved meeting house, where each side delivered speeches backed by rousing waiata.
The formalities inside the meeting house were displayed on a large screen for those watching outside. The visitors spoke in English, Gaelic and Welsh, and sang the English patriotic song Jerusalem, the Irish folk ballad Fields of Athenry and the Welsh hymn Calon Lan.
Mr Spencer said it was a privilege to be part of such an extraordinary welcome.
"This is a welcome the British and Irish Lions will remember for the rest of their lives," he said.
Treaty Grounds cultural manager Mori Rapana, chief organiser of the powhiri, said welcoming the Lions to Waitangi, the very spot where the British and Maori signed the Treaty, had special significance.
"It was exhilarating, exciting ... There was a lot of wairua, the ground was thumping, the energy was flowing. The Lions said they were blown away."
With help from volunteers from as far away as Auckland, Kaitaia and Sydney, he had been confident of pulling off a powhiri on yesterday's scale.
Treaty Grounds manager Greg McManus hoped the "spectacular" welcome, with a turnout of thousands boosted by fine weather, was the first of many such events at the Treaty Grounds.
Among the dignitaries present were the Whangarei and Far North mayors, a bevy of politicians, and the British High Commissioner.
Afterwards the Lions spoke about the latest terror attacks in London, which many only heard about as they emerged from the meeting house.
Mr Warburton said the team would dedicate its efforts on the tour to those who had been affected.
"I guess we can play a small part in trying to cheer a majority of the nation up by trying to be successful over here," he said.
Mr Spencer said the team was four nations but united in one jersey and in their sentiments about the "dreadful tragedy" in London.