The first prosecution witness is giving evidence at Kenyan Deputy President William Ruto's trial at the International Criminal Court (ICC).
Mr Ruto has been charged with crimes against humanity, following allegations that he orchestrated violence after disputed elections in 2007.
Mr Ruto, the first serving official to appear at the ICC, denies the charge.
The court ruled the witness's identity, an alleged survivor of a church arson attack, should not be revealed.
This was for her own safety and she will be known by the number 536.
She is giving evidence from behind a curtain - on the court video her image is pixelated and voice distorted.
Mr Ruto is watching on her left, but she is hidden from him.
Chief prosecutor Fatou Bensouda has previously said that witnesses have been intimidated in an attempt to prevent them from giving evidence.
Kenya's President Uhuru Kenyatta is due to stand trial in November. He also denies charges of fuelling violence after the 2007 election.
Some 1,200 people were killed and 600,000 forced from their homes in weeks of violence after the election.
More than 40,000 people are estimated to be still living in camps, which Mr Kenyatta has promised to close by 20 September.
He and Mr Ruto were on opposite sides during the 2007 election, but formed an alliance to win elections in March this year.
Analysts believe the ICC charges bolstered their campaign, as many Kenyans accused the court of interfering in Kenya's domestic affairs.
Prosecutor Anton Steynberg told the court that "22 victims and witnesses, common Kenyan people, who will describe the attacks" would be called to testify in Mr Ruto's trial, Kenya's Daily Nation newspaper reports.
Mr Ruto is being charged alongside journalist Joshua arap Sang.
He is the head of a Kalenjin-language radio station and is accused of whipping up ethnic hatred - a charge he denies.
The first witness was a victim of an arson attack on New Year's Day 2008 on the Kiambaa Church in Kenya's Rift Valley, one of the areas worst-affected by the violence.
A mob set ablaze the church where people were taking refuge, burning 36 people beyond recognition, correspondents say.
The witness, a member of the Kikuyu ethnic group, told the court that thousands of people surged towards the church, chanting, their faces disguised with white clay, many wearing cloths and bandanas.
They were armed with traditional weapons, machetes, axes and sticks, she said.
She was among hundreds of people who had earlier fled to the church, with mattresses and blankets, after members of the rival Kalenjin ethnic group warned of consequences if they did not vote for the Orange Democratic Movement (ODM), the woman said.
Mr Ruto was at the time a member of the ODM, led by Raila Odinga who later became prime minister in the unity government that followed the violence.
"The door [to the church] was barricaded with bicycles, and we were all trying to escape. I threw my child out of the window," Reuters news agency quotes the witness as saying.
"When somebody tried to leave the church, they would grab the person and push them back inside," she said, the AFP news agency reports.
The violence erupted after the ODM said it had been robbed of victory
It then took ethnic overtones, with Kenya's main groups - especially Kikuyus and Kalenjins - involved in attacks and reprisal attacks.
When Mr Ruto appeared in court last week, Ms Bensouda accused him of forming an "army" of Kalenjin youth to fight for power.
Mr Ruto's defence lawyer, Karim Khan, accused the prosecution of building its case on "a conspiracy of lies".