He said a celebration of Cox's life and values would be held on Wednesday across the world, including London, New York, Brussels and the Middle East.
After the tributes in the House of Commons and House of Lords, politicians were to attend a memorial service at St. Margaret's Church, according to the British Broadcasting Corporation (BBC).
Suspect Thomas Mair, charged with murdering Cox, appeared at the Old Bailey, the central criminal court of England and Wales, on Monday afternoon. According to local media, he appeared via video link from Belmarsh Prison, and the hearing only lasted a few minutes.
He answered his correct name when asked, rather than how he answered on Saturday when at Westminster Magistrates' Court: "Death to traitors, freedom for Britain."
He will appear at the Old Bailey at 10 am local time (0900 GMT) on Thursday.