Hamren named a 23-man squad to face the Portuguese in the two-legged playoff, with the first match in Lisbon on November 15 and the return leg in Stockholm four days later.
"Portugal are clear favourites, but there are possibilities. Our ambition is to go to the World Cup," Hamren told reporters at the Friends Arena on Tuesday.
"When they create chances we must have the margins on our side.
"There's a lot that has to work and it's a big challenge. If we make a mistake they'll punish us immediately - that's the level of this opponent."
With comparisons being drawn between Cristiano Ronaldo and Sweden striker Zlatan Ibrahimovic, Hamren said his side had a plan to stop the Portuguese winger, but that he was not the only player they needed to worry about.
"Ronaldo is the player that can do that little bit extra, like Zlatan can do for us. It's about stopping Ronaldo before he gets up to speed, but we can't just focus on one player when we meet a top team like Portugal."
Squad
Goalkeepers: Andreas Isaksson (Kasimpasa SK), Johan Wiland (FC Copenhagen), Kristoffer Nordfeldt (SC Heerenveen)
Defenders: Pierre Bengtsson (FC Copenhagen), Mikael Antonsson (Bologna), Per Nilsson (Nurnberg), Jonas Olsson (West Bromwich Albion), Andreas Granqvist (Krasnodar), Martin Olsson (Norwich City), Adam Johansson (IFK Gothenburg), Mikael Lustig (Celtic)
Midfielders: Rasmus Elm (CSKA Moscow), Pontus Wernbloom (CSKA Moscow), Anders Svensson (Elfsborg), Alexander Kacaniklic (Fulham), Kim Kallstrom (Spartak Moscow), Sebastian Larsson (Sunderland), Jimmy Durmaz (Genclerbirligi), Erkan Zengin (Eskisehirspor)
Forwards: Johan Elmander (Norwich City), Ola Toivonen (PSV Eindhoven), Zlatan Ibrahimovic (Paris Saint Germain), Alexander Gerndt (Young Boys)
(Reporting by Philip O'Connor, editing by Justin Palmer)
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