A woman was attacked near St Anne's Church in Spitalfields - where friends had gathered to mark Jean Charles de Menezes's 28th birthday on Saturday.
They heard her screams and ran out to help, while her attacker went back to his van and grabbed a metal bar.
He then smashed the door of the church before driving off.
The attack happened at about 1940 GMT, as the woman walked close to the church where the service was under way.
Two arrested
The man jumped out of a red van and tried to grab her handbag - but those at the service came to her aid.
He made his escape with a woman driver, after attacking the church doors. A man and woman were later arrested in east London.
Friends and relatives of Mr Menezes, who was killed at Stockwell Tube station on 22 July - the day after the failed bomb attacks - had gathered to mark what would have been his birthday.
His parents flew to Britain to visit the scene and launch the Justice 4 Jean campaign, but have since returned to his native Brazil. A reading was given on their behalf during the service.
The Independent Police Complaints Commission (IPCC) is investigating Mr Menezes's death, and has said the officers involved could face charges.
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