“当场”常见的英文表达有 on the spot、right there and then 或 immediately at the scene，具体使用可根据语境选择：
1、 On the spot
释义：在现场；立刻；当场。
例句：
The police arrested him on the spot.（警察当场逮捕了他。）
When asked about the incident, he denied everything on the spot.（当被问及此事时，他当场否认了一切。）
2、 Right there and then
释义：立刻；当场。
例句：
I decided right there and then to quit my job.（我当时就决定辞职了。）
She realized her mistake right there and then.（她当场就意识到了自己的错误。）
3、 Immediately at the scene
释义：在现场立即；当场。
例句：
The witness provided a statement immediately at the scene.（目击者在现场立即提供了证词。）
The paramedics administered first aid immediately at the scene.（医护人员当场进行了急救。）