Defeated 2000 Tory candidate Steve Norris has secured backers including Golin/Harris International PA head Alex Challoner and Bell Pottinger PA account director Malcolm Gooderham.
Press work for his Tory nomination rival, London Assembly member Roger Evans, will be handled by Evans' personal assistant - and campaign manager - Patricia Scott.
Lib Dem members in London are to choose from three candidates: Simon Hughes, Susan Kramer and Donnachadh McCarthy.
Hughes has turned to former Radio 1 press head Paul Simpson, who this month launched his own PR agency - Dutch.
Kramer has split PR responsibilities between herself and Lambeth councillor Ashley Lumsden. McCarthy - who ran PR for Southwark council's Lib Dem group between 1994 and 1998 - is now handling his own media relations.
Labour's official candidate - deputy mayor Nicky Gavron - is supported by the press office at the party's recently acquired Old Queen Street HQ.
The Green Party is expected to select its candidate by April. Defeated 2000 candidate Darren Johnson faces competition from Hugo Charlton, who has hired PR consultant Birgit Cunningham.
Ken Livingstone is not expected to begin campaigning until four weeks before the election, scheduled for May or June 2004.