Porto is the second biggest city of Portugal.
For the airport of Porto, check out the Francisco de Sá Carneiro article.
Going South can be hard, even when you start from within the nearby city of Gaia, which is just across the Douro River. One spot in Gaia is a roundabout, next to the last metro stop (Santo Ovídio). This is an okay spot to stand, even at night, with plenty of room for cars and trucks to pull over.
If you don't want to stand at the roundabout, a better option is to walk from there onto the first petrol station. Its just a bit more than 2km and you can start hitching right on a petrol station on the motorway. Just walk down the hill for 2km as indicated on the map, then you cross the motorway through a tunnel. Walk up a smaller hill for 200m until on your left there is a spring and a smaller sandy road. On both sides of the sandy road there are two wild gardens. On the end of the road (heading to the motorway) up comes a house on your left and a door on your right. The door leads to the petrol station framed by a 1.80 metre high fence on top with barb wire. The fence has already been repaired quite a bit. After 50m you come to another metal door which leads you onto the petrol station. It can be opened with a little rusty wire on the right - just pull very hard. Otherwise you can always shout for somebody to open the door. (August 2011)
Better option: 100m before the gas station there's a road passing under the highway - it's an easy place to go up into the highway, and just walk your way until the gas station. In order to reach easily this point, and eventually go up into the highway it's also possible to catch a bus from the main bus station in BATALHA with destination PEROSINHO and get off at LABORIM stop (just ask the driver). The bus will drop you just a few meters from there.
Look for: Rua da Palmeira, Vila Nova de Gaia, Portugal
North towards Braga and Spain (Vigo, Santiago de Compostella)
Option 1 Take the Tram/Train to São Frutuoso (less than 2 Euro). Walk the Rua Prof. Antonio Marques until the end and you'll arrive at a petrol station right next to the motorway to Vigo/Spain after appx 800m. Note: There is a fence around the whole petrol station, almost impossible to climb. To ring the bell and ask for entrance is a bad idea. Walk along the fence until the bridge where it's easy to climb over (January 2012). Try to be unwatched climbing the fence, the petrol station is privatized. I got kicked out in January 2012...In February 2012 I got dropped off on the other side direction Lisbon and had no problem at all staying at the petrol station. Just make sure they don't see you enter the petrol station "illegally".--Wukk 18:14, 28 February 2012 (CET)
Option 2 You may hitch from the petrol station close to metro station "Estadio do mar". At the roundabout, take Av. Calouste Gulbenkian, pass the School of design and you will see an entrance to coastal motorway A28 going North. People might also be going North using other motorways from there since there is a junction heading East close by.
There are probably many other ways of leaving Porto in that direction, yet to be discovered and shared.
Me and my travel-buddy tried the petrol station close to the metro station "Estadio do mar", but couldn't get a ride after trying for one hour. Some people at the petrol station told us about a FREE shuttle bus going north to IKEA (located close to the motorway A28) where a lot of spanish people go for shopping. The best way to get to the bus-station for the free shuttle-bus is to take the metro to "Senhora da Hora" (one stop before "Estadio do mar"), walk back (if you took the metro from the city) on the street along the metro-line, take a right into the street called "Avenida de António Domingues dos Santos / Rua da Barranha" and find a bus-station on the right side. This is how it looks like:
Take the red bus to "Mar Shopping" and get off there. We were standing at the exit of the parking-lot of IKEA and got a ride to "Viana do Castelo" after 15 minutes. This is how the mall and the bus look like:
The free shuttle-bus runs every 1,5 hours. The IKEA is open Monday to Saturday 10h-23h and Sunday/Holidays 10h-20h. I assume those times the shuttle bus runs aswell. Should it not run you can walk down "Avenida de António Domingues dos Santos / Rua da Barranha" from the bus-station to the metro station "Estadio do mar" in a few minutes and follow the descriptions above.