* Polls tip centrist Macron as likely winner
* But Melenchon has made ground in polls in last 2 weeks
* Big rallies set for Monday
* Authorities on high security alert for vote
* Hollande says he would feel "guilty" if Le Pen won
By Johanna Decorse and Emmanuel Jarry
TOULOUSE/PARIS, April 16 (Reuters) - Far-left candidate Jean-Luc Melenchon drew tens of thousands to a rally on Sunday, aiming to maintain momentum that has pushed the one-time outsider into contention in the French presidential election, with the first round of voting a week away.
Opinion polls show the top four candidates - centrist Emmanuel Macron, far-right leader Marine Le Pen, conservative Francois Fillon and Melenchon - within touching distance of each other before the April 23 first round after a surge by Melenchon in the last few weeks.
The polls tip Macron to beat National Front leader Le Pen in a runoff between the two leading candidates on May 7.
The outside possibility of a second-round showdown between Le Pen and Melenchon, both of whom propose a referendum on France's membership of the European Union if their attempts to overhaul the bloc fail, has raised concern among some investors after last year's shock British vote to leave the bloc.
Melenchon has pledged sharply higher government spending and would impose a 90 percent tax on top earners and take France out of NATO. Le Pen has promised a crackdown on illegal immigration and wants to scrap the euro.
"We represent a France of beauty and generosity, that starts each new day mindful of our motto - liberty, equality and fraternity," Melenchon told the Toulouse rally. His team said 70,000 turned up while local police put the number at 40,000.
Outgoing Socialist President Francois Hollande told a French television programme broadcast on Sunday that he would personally feel "guilty" if Le Pen became president, bringing a far right party to power in France.
Front-runner Macron, a former economy minister and investment banker who has attempted to break the mould of French politics by launching a new centrist party, opted for a lower-profile visit in the Paris area on Sunday to people with difficulties finding housing.
Fillon, who is being investigated by magistrates on suspicion of embezzling state funds, held meetings with Catholic voters on Saturday but had no rally on Sunday, while all four leading candidates are set to hold large rallies on Monday.
The elections take place with France on heightened security alert following a spate of attacks by Islamist militants since January 2015 that have killed more than 230 people.
Asked by the Journal du Dimanche newspaper if authorities feared an attack timed to coincide with the election, Interior Minister Matthias Fekl replied: "No threat is being ruled out."
Anti-fascists held a demonstration in Paris on Sunday, and Fekl said authorities were aware of the risk of violent street protests by opponents of the National Front if Le Pen got through to the second round.
(Additional reporting by Sudip Kar-Gupta and Myriam Rivet; Editing by Adrian Croft)