Before the election, Kalenjin youths also had threatened Kikuyus, she said.
She told the court a local political candidate was in the mob carrying a jerry can and that the youths barricaded the church doors shut with bicycles, threw stones and fired arrows at the windows and then torched the building on Jan. 1, 2008.
"We were all trying to find a way to escape," she told judges. "I was carrying my small child . and I tried to escape. I threw the child out of the window."
Presiding judge Chile Eboe-Osuji adjourned the trial early for a 90-minute lunch break as the witness was overcome with emotion. She was due to continue her account of the massacre in the afternoon session.
The witness did not directly tie either Ruto or Sang to the church attack, but prosecutors say both defendants were part of a Kalenjin conspiracy to attack perceived supporters of their political rivals after the election.
Both Ruto and Sang portray themselves as men of peace who tried to ease ethnic tensions around the election.
At elections earlier this year, Ruto joined forces with former political rival Uhuru Kenyatta and the pair were elected deputy president and president respectively.
ICC prosecutors also have charged Kenyatta with involvement in violence after the 2007 election. He is due to go on trial in November.