The party's conference was kicked off with a festival style gathering in Brighton Park, with supporters chanting the leader's anthem, "oh Jeremy Corbyn".
A party spokesman said the preparations were for events that could occur under any government.
But McDonnell criticized the Conservatives' economic track record since they took power from Labour in 2010 and said they were responsible for slow wage growth which was forcing consumers into debt and risking another economic crisis.
McDonnell said he wanted to get this underway quickly in the event Labour is voted in to government.
Opponents said that could be very expensive, but Labour insist the taxpayer will make savings. Although no election is due until 2022, it is preparing for Theresa May's Conservative minority government to fall much sooner.
A run on the pound refers to a situation in which global investors and market speculators sell sterling and sell sterling assets en masse, forcing its value down. Successive governments have attempted to reform the process.
Such proposals will no doubt go down well with some voters, however the defining political issue of our time - Brexit - is the one that is arguably the most problematic for Labour given the sharp differences of opinion that exist between the leadership and some of the party's high-profile MPs.
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The Corona Fire Department on Monday night tweeted that the fire was up to 2,000 acres and 5% contained. Overnight, five helicopters with night-vision and three planes tried combating the wildfire from above.
Speaking during the party conference in Brighton, McDonnell said Labour were "scenario-planning" for a drop in the value of the currency in order to "hit the deck running" following a prospective electoral victory.
"What if there is a run on the pound?" What happens if there is this concept of capital flight? He told delegates: "I am calling upon the government to act now and apply the same rules on payday loans to credit card debt". And let's start doing the detailed work now.
Labour's conference has been marked by the party facing a fresh anti-Semitism row, following reported remarks on Holocaust denial by a speaker at a fringe meeting.
Several business groups have expressed concern related to the speech.
He also said Labour would have to resist pressure from senior civil servants, "Sir Humphreys", who wanted to preserve the status quo. And we got to that position. The PFI model of funding public services such as new schools and hospitals was established by the Conservatives in the 1990s and has continued under successive governments.
In the run-up to his re-election previous year she tweeted: "One day in the far distant future we'll look back and we won't laugh, because this isn't bloody amusing". It also said that the last thing the industry wants is for vulnerable people forced to deal with "unscrupulous and unregulated lenders".