Abortion is strictly forbidden. Here is what this chapter of the Gautama Dharma Sutras says:

A woman becomes an outcast by procuring abortion, by connection with a (man of) lower (caste) and (the like heinous crimes).

And here is what this chapter of the Manu Smriti says:

Libations of water shall not be offered to those who (neglect the prescribed rites and may be said to) have been born in vain, to those born in consequence of an illegal mixture of the castes, to those who are ascetics (of heretical sects), and to those who have committed suicide, To women who have joined a heretical sect, who through lust live (with many men), who have caused an abortion, have killed their husbands, or drink spirituous liquor.

There is also punishment for men who cause abortion, as described in this chapter of the Vishnu Purana:

He who causes abortion, plunders a town, kills a cow, or strangles a man, goes to the Rodha hell[.]

And here is what this chapter of the Apastamba Dharma Sutras says:

Now (follows the enumeration of) the actions which cause loss of caste (Patanîya). (These are) stealing (gold), crimes whereby one becomes an Abhisasta, homicide, neglect of the Vedas, causing abortion, incestuous connection with relations born from the same womb as one's mother or father, and with the offspring of such persons, drinking spirituous liquor, and intercourse with persons the intercourse with whom is forbidden.

And for good measure here is what Vidura says in this chapter of the Udyoga Parva of the Mahabharata:

One that setteth fire to a dwelling house, an administerer of poison, a pander, a vendor of the Soma-juice, a maker of arrows, an astrologer, one that injureth friends, an adulterer, one that causeth abortion, a violater of his preceptor's bed, a Brahmana addicted to drink, one that is sharp-speeched, a raker of old sores, an atheist, a reviler of the Vedas, and taker of bribes, one whose investiture with the sacred thread has been delayed beyond the prescribed age, one that secretly slayeth cattle, and one that slayeth him who prayeth for protection,--these all are reckoned as equal in moral turpitude as the slayers of Brahmanas

And to show how serious killing a foetus is, here is what Bhishma describes as the repentance for it in this chapter of the Shanti Parva of the Mahabharata:

One guilty of foeticide becomes cleansed if he dies of wounds received in battle fought for the sake of kine and Brahmanas. He may also be cleansed by casting his person on a blazing fire.

On the other hand, this chapter Devi Bhagavatam prescribes an easier expiation:

In this way if the Gâyatrî be repeated one thousand times, the sins arising from killing a cow, father, mother, from causing abortions, going to the wife of one’s Guru, stealing a Brâhmana’s property, a Brâhman’s field, drinking wine, etc., all are destroyed.

But if that's the case it just goes to show the extreme power of the Gayatri mantra, not the lack of severity of the sin.

Also, you asked about what modern-day figures have said. Here is what ISKCON founder Srila Prabhupada says about abortion:

They are killing the baby in the womb. How cruel! In this age of unwanted population, man is losing his compassion. When you kill a living entity, even an ant, you are interfering with its spiritual evolution, its progress. That living entity must again take on that same life form to complete its designated life term in that body. And the killer must return to pay for damages.

On the other hand, Osho Rajneesh says abortion is a virtue in this chapter of one of his books:

Abortion is not a sin - in this overpopulated world, abortion is a virtue. And if abortion is a sin then the Polack Pope and Mother Teresa and company are responsible for it, because they are against contraceptives, they are against birth control methods, they are against the pill. These are the people who are the cause of all the abortions, they are responsible. To me they are great criminals.

But Osho Rajneesh is known for his unorthodox views. Finally, here is what Sathya Sai Baba said:

When asked the question, “When does the soul enter the body?”, Swami replied, “The soul enters the body on or after the fifth month.” When asked about abortion after the fifth month, He answered: “No abortion!”. When asked about abortion before the fifth month, Swami said, “No! No! Even before - abortion is bad... Mind control... Use mind control."

I think that means restrain your mind and practice abstinence, but perhaps he's talking about psychic abortion!